the humming was a little more noticeable in the helicopter cabin, but not much. john mclaughlin, sprawled as comfortably as his two meters of height would permit in its confines, had noticed the sound only at first; and after remarking to himself that they seemed to be building better ion turbines since he had left earth, had permitted his thoughts to wander in other directions. these did not concern felodons; the interest there was not, at the moment, mutual. the rather crowded cabin offered material enough for consideration.

mclaughlin was not a scientist by training, but neither was he the sort of guide that might have been found in yukon or amazon territory a few centuries back. he did not despise people merely because they were, by his standards, greenhorns. he knew that each of the other men now sharing this cabin with him was an expert in his own field, even though none of them, in spite of his training, would have been able to survive for more than a day in the jungles of viridis. after all, why should they have learned such an art? there were other things worth learning, and one could always hire mclaughlin if a need to visit the jungles developed. since this particular party had done just that, they were evidently a fairly practical crew.

they were not talking very much, which from the guide's viewpoint was an additional point in their favor. they already knew what they planned to do, and saw no point in repeating what had already been said. of course, if they should fail to find the area they were seeking, there would be talk—all of it aimed at mclaughlin; but he had no fear on that score. there were few enough mountains on viridis, and of those few by far the greater number were volcanic cinder cones. when these scientists had specified a region of tilted-block or folded mountains, the guide had been more than dubious at first. it had taken him time to recall that there was a small area meeting these specifications less than fifteen hundred miles from the spaceport at emeraude. he was not himself a geologist, but pictures and diagrams had been used freely in explaining to him just what was wanted, and he was quite certain that the party would be satisfied with what he had to offer.

a slight rocking in the hitherto steady motion of the helicopter roused him from this line of reverie. they were already several hours from emeraude, and mclaughlin realized that he should have been paying more attention to the course. he straightened up in his seat and looked out.

to the left and ahead was a huge thunderhead, whose satellite air currents had probably caused the variation on the helicopter's flight path. more important, there was land in sight. mclaughlin knew that the long flight across green bay was over. he waited, however, before saying anything. he had given the pilot full instructions as to the route before take-off, and he wanted to see whether those had been clear enough.

apparently they had. without asking questions or even looking back at the guide, lampert swung the aircraft from its northerly heading onto one which paralleled the shoreline, a turn of about forty-five degrees to the right, and the helicopter resumed its steady flight.

mclaughlin did not relax. from now on the route was a little more difficult to follow, and there were not too many more hours of daylight. the shadowless night glow which made vision relatively easy after sunset did not lend itself to aerial navigation over a very poorly mapped world. he kept his eyes on the shoreline, watching for the landmarks he had not seen for many months—and then not from above. he did not see the felodon which became so intensely interested in the helicopter. if he had, he would have attached little importance to the creature's presence, and he could not possibly have seen its actions in sufficient detail to catch any peculiarities in them.

no one else saw the beast, either. the change in course had roused most of the party from whatever lines of thought they had been pursuing, as it had mclaughlin, and most of them were looking out the windows; but they were interested in what lay ahead, not below. sometime soon the relative monotony of jungle and swamp should be relieved by rising ground, indicating the nearness of the mountains they sought; and the helicopter's flight altitude of some two thousand feet was low enough to permit any significant rise of terrain to be visible. sulewayo, the younger paleontologist, made a remark to that effect, which passed without comment. real conversation did not start for some minutes.

"as i understand it, we have one more course change before we see the mountains. isn't there a river we have to follow for a time, string?" lampert asked the question without looking back.

"that's right," mclaughlin replied. "it runs into green bay from almost straight north, and about a hundred miles inland makes a turn to the east. that's general direction. it winds a lot."

"it would, in country as nearly peneplaned as this," muttered lampert under his breath.

"the mountains you want start about sixty airline miles from the big bend. if you trust your gyro compass enough, you can head for them directly from the river mouth. if you have any doubt about being able to hold a line, though, follow the river. i doubt that there are any good landmarks otherwise. of course, i've only seen the area from the surface and close to the river, but i'd be very surprised if there was anything around but the swamp-and-jungle mess we're over now."

"so would i. we'll stay in sight of the river, but edge as far east as visibility lets us." the guide approved this plan with a nod, and the conversation lapsed for several minutes. the silence was finally broken once more by sulewayo.

"i hope these hills we're looking for have something of interest. this planet is the most monotonous i've seen yet. where it isn't jungle it's swamp; and the only difference between the two is that the jungle grows higher trees." mclaughlin's face crinkled into something like a smile, and he sat up once more.

"there's one other difference," he remarked.

"what's that?"

"in the jungle, dressed and equipped as you now are, you might live as long as a day. in the swamp, five minutes would be an optimistic estimate." sulewayo looked down at the shorts and boots which constituted his costume, and shrugged.

"i admit the point, but i don't expect to go out this way. what i actually wear and carry, beside my professional equipment, is up to you. also, i was referring to appearances. beta lyrae nine looked almost as dull as this world from above, and i'll bet it was as least as deadly when you reached the surface." mclaughlin had never visited new sheol, and admitted it, but it took more than that to stop sulewayo.

"actually, i was hoping that these hills didn't turn out to be so covered with soil that any fossils would be yards underground at the best. do you recall any places where the rock strata themselves were exposed—steep cliffs, or deep stream gullies, perhaps?"

"definitely yes. the big river cuts right across the range, or else starts in it. it comes out from a canyon like that of the colorado on earth, though a lot less spectacular. actually i don't know anything about the country more than a couple of miles up that canyon. i was stopped on the river by rapids, and couldn't get my amphib out on either side. for the most part there simply wasn't any shore, just cliff."

"quite a current, i suppose?" lampert cut in.

"actually, not very much. i went swimming in worse, on earth."

"that hardly ties in with steep cliffs and a river cutting through a mountain range."

mclaughlin shrugged. "you're the geologist. look it over for yourself. maybe you'll just have to add it to the list of things you don't understand about viridis."

"fair enough." the pilot-commander-geophysicist nodded. "i did not mean to imply that you were not reporting accurately; but the situation you have described would be a trifle queer on more planets than earth, i assure you. still, with luck your cliffs will show fossils. maybe we'll solve one problem in exchange for another. life could be worse."

"just hope we don't solve the first one by proving that certain geophysicists have been talking through their hats," the hitherto silent krendall remarked.

"eh?"

"what would you do if we found a chunk of, say, pegmatite with radioactive inclusions that checked out at half a billion years instead of the thirty-odd million you lads have been giving us as a time scale for this mudball?"

"i should check very carefully under what circumstances and in what location you found it. if necessary, i would admit that the problem had disappeared. half a billion years would account reasonably well for the evolutionary status of this planet's life forms, though actually it took earth a good deal longer to reach a corresponding condition. frankly, however, i do not expect any such find. we spotted our borings rather carefully, and should have taken pretty representative samples."

"i'm sure you did. if your results are right, it just means that the problem belongs to hans and me—and string here had better find us a lot of fossils."

"you'll have to find your own bones," mclaughlin replied. "i'm taking you to the sort of ground you want. a fossil would have to show its teeth in my face before i'd recognize it—and then i'd probably shoot before i realized it was dead."

"all right," sulewayo chuckled. "you take care of the quick, and krendall and i will worry about the dead. dr. lampert can figure out how old the fossils are if we find any, and take can look for stone axes."

"or automobiles, or pieces of space-drive tubes, or other artifacts," mitsuitei answered the implied dig. "i plan to sit back and loaf, unless and until one of you lads turns up a skull that could have held more than half an ounce of brain. i am going to be very unscientific. i believe that there is nothing on this planet for an archeologist to do, and i am not going to work myself into a lather to prove myself wrong."

"you've formed an opinion rather early in the game," lampert remarked. "after all, remarkably little of this world has been explored. why should there not be traces of occupation in unknown areas such as we are about to visit?"

"because, while most of the planet remains unexplored, a very large number of places which should have furnished traces of habitation have failed to do so. we've surveyed many spots which were, or are, ideal for cities based on ocean commerce, or market centers for what could be farm areas, or spaceports. after a while you get to a point where such finds can be predicted with some certainty. as i said, i am far from certain, and it would be most unreasonable to say i was; but in the area we are seeking, i see no reason to expect anything of interest to my profession."

lampert shrugged and brought his full attention back to the controls. the sun was slowly sinking, bringing into bolder relief the irregularities of the ground as their shadows lengthened. however, these irregularities were still few, and the jungle roof was for the most part evenly illuminated. as mclaughlin had expected, there was nothing that could be used as a landmark. in its own way, the forest was as featureless as the ocean. the pilot kept his gaze riveted ahead, in expectation of the river which the guide had told them to expect; and presently he saw it. reflecting the color of the faintly purplish sky, it stood out fairly well against the gray-green of the jungle, once they were close enough to penetrate the ever-present haze.

with mclaughlin nodding silent approval, lampert swung the helicopter to the left and proceeded more nearly straight north, angling gradually toward the river. now the jungle took on a little more feature, though still nothing that could be used for guidance. at fairly frequent intervals a glint of water became visible through the trees directly below them. evidently numerous tributaries were feeding into the larger stream; but none of these could be seen from any distance. for the most part they were so narrow that the trees growing on each side met above them.

"i should think that one could cover a great deal of that territory in a boat," remarked mitsuitei, after nearly half an hour in the new direction.

"you'd need an amphib," replied the guide. "a boat is all right for the main stream, but all that stuff coming in from the sides is so shallow that you'd never make progress with anything else. i've tried most of them in my own croc. every time i've had to crawl rather than float before i was a mile from the river."

"how is the ground? swamp?"

"no, it's fairly solid for the most part. it doesn't show very well yet even with the sun as low as it is, but the general ground level is pushing up slowly all along here. we'll be in sight of your mountains before too long."

this declaration brought all members of the group to the windows, all five pairs of eyes covering the quadrant of vision below and ahead. the meandering river was now on their left, but just visible through the haze ahead of them was the eastward turn mclaughlin had predicted. lampert headed a little more to the right in an attempt to cut the final corner, but the helicopter reached the winding purplish band before their goal came in sight in spite of this effort. the flyer hummed on.

the bars of sunlight admitted by the side ports had been nearly horizontal when the turn to the east cut them off. they were only slightly more so when mclaughlin gave a satisfied grunt, and nodded forward. the others followed his gaze.

straight ahead, little could be seen because of the "bright spot" familiar to every flyer—the shadowless area directly opposite the sun, centered on the aircraft's own shadow. to either side, however, the promised hills rose out of the jungle to heights exceeding the present flight altitude of the helicopter. presumably the canyon from which the river was supposed to emerge lay in their path. so, at any rate, lampert remarked; and mclaughlin confirmed him.

"i'd cruise pretty slowly from here on," the guide added. "there are a number of hills on this side of the range. even if you're not worried about running into one of them, you may want to examine them for exposed rocks."

"mightn't it be better to find a spot to park before the sun goes down?" countered the pilot.

"it might. what i said still holds, though. you haven't much chance finding one inside the canyon without quite a long search, and it will be best to stay this side of the range until sunrise. remember my trouble in finding a beach for the amphib while i was inside."

"all right. can we land in jungle, though?"

"not unless you want to fold the blades in flight and drop the last twenty to fifty feet. hunt for a fairly high hill. they're usually somewhat bare on top, and you'll at least have room for the rotors to swing. if you don't like that, or can't find a suitable hilltop, land on the river and tie up to the shore—but again, don't try that in the canyon. you're unlikely to find anything to tie up to."

"this machine has good lights, i suppose you realize—but then, you know the planet. as far as i'm concerned, what you say goes. are the chances of a hill equally good on either side of the river?"

"maybe a little better to the north. the ground looked higher that way when i came out of the canyon." lampert obediently eased the flyer's course a trifle to the left, and everyone aboard watched the ground as it began to rise toward them.

at first the "hills" were merely low mounds, as jungle-covered as the level ground; but very quickly these gave way to higher, steeper rises on whose tops the larger trees grew very sparsely. one of these was quickly selected after a brief, questioning glance from lampert to the guide, and the helicopter began to descend.

"we'd better take what we have now." mclaughlin amplified the nod with which he had answered the pilot. "this belt of hills is pretty narrow, and we'd be into the main range in another minute or two."

"do you know whether the other side is as abrupt, or whether—" lampert's question was cut short by an exclamation from mitsuitei.

"rob! hold it a moment!"

lampert was a good pilot; the increase in rotor-blade pitch under his deft fingers brought the helicopter's descent to as nearly an instant halt as was possible to anything airborne. not until he had also checked horizontal drift did he look in the direction the archaeologist was indicating. by then, everyone else had seen what had attracted mitsuitei's attention.

between the hill on which lampert had intended to land and the river were several lower eminences. these were now almost directly south of the helicopter, and every detail upon them was shown in exaggerated relief by shadows stretching to the east. it was one of these hills which mitsuitei was examining with the utmost care.

it was covered with jungle, like the rest; but a curious regularity was visible. the trees appeared, at this distance, to be of the usual species; but some of them towered over their fellows by a good thirty or forty feet.

this in itself was not odd. the whole jungle was studded with such projections. however, on this hill the taller trees seemed to have been planted in orderly rows. five solid lines of them were visible, extending roughly north and south so that their long shadows made them stand out sharply. they were separated from each other by perhaps a quarter of a mile. running at right angles to them were other, less outstanding rows of vegetation. lampert was not quite sure that these were not the product of his own imagination, since the trees which formed them rose little if any above the general level. the whole hilltop, however, suggested something to every man who saw it. the archaeologist was the first to give voice to the impression.

"that was a city!"

no one answered. some of the scientists must have thought that he was jumping from one opinion to its direct opposite on the strength of some rather feeble evidence; but the thought went unvoiced. they simply looked—except for sulewayo, who moved to turn a camera on the scene.

"rob! can we land there? now?" lampert had anticipated this question, but could have answered it without hesitation in any case.

"sure—if you don't mind using string's method of folding the blades and falling in." the archaeologist turned to the guide.

"will it be hard to get there on foot from this hill we're heading for?" mclaughlin shrugged.

"from two hours to a day, depending on undergrowth."

"we have torches. we can burn our way if the vegetation is dense."

"half a day, then. you'll still have to let the steam clear pretty often. there's little wind below the trees, and the air is saturated."

"well, that place will be worth more than a day of anyone's time. maybe tomorrow we can—"

"hold up a moment, take!" lampert cut in, before mitsuitei could develop his plan further. "if you take string out to that hill before take-off tomorrow, what do the rest of us do for the day—or week—before you get back? what we'd better do is note this place, go on to the canyon, set up camp, get the fossil hunting going, and after our routine is set up and we know the more common dangers of the neighborhood, perhaps we can spare mclaughlin for a day or two so that you can look over your city—if that's what it is."

lampert's last few words banished the hurt expression from the little man's face.

"what do you mean—if? what else could make a pattern like that? it must have been streets."

"or a joint system in the rock below, trapping enough water—or draining enough off—to permit superior growth along the joint lines. or a system of tilted strata doing the same thing—"

"if it's the latter, it's just the sort of thing you want, too. it should bring fossils near the surface."

the pilot nodded slowly. "you do make it sound more attractive. still, i think we'd better follow the original plan, except that i may come with you myself when we do get around to looking that hill over." he turned back to the controls and resumed their descent. mitsuitei subsided once more to his seat. the archaeologist realized the wisdom of lampert's decision, but did not particularly enjoy the enforced wait. his face showed the fact, until sulewayo opened the camera he had been using and passed him the sheaf of prints on which the "city" appeared. as the young paleontologist had expected, these so occupied the little man's attention that he did not even notice the landing.

the helicopter settled to the hilltop which lampert had chosen, in the center of a quadrangle of trees growing just far enough apart to give clearance to the rotors.

the sun was nearly gone. it had vanished in the haze as they dropped below flight altitude. mclaughlin knew that in all too short a time it would be as dark as viridis ever became. the nights could be dangerous. there was quite enough light to deceive a man into thinking he could see clearly, and an inexperienced wanderer might not realize until too late that details were not really distinct and that there was no clue to direction in the shadowless glow. mclaughlin himself could use the moons, but he doubted that any other member of the party could do so. they—or their motions—took knowing.

he was pleased to note that there was no general rush to the door as the great blades whistled gently to a stop. the scientists turned to him, but remained where they were. no words were spoken, but lampert's relinquishment of command was evident. mclaughlin unfolded his length from the seat.

"there are two choices," he said. "we can sleep in the 'copter, or outside. the first will be a trifle cramped, but the second will require either a double circle of charged wire or two armed guards on constant watch. with no offense meant, i doubt that anyone but myself in this group could qualify as a night guard."

"why a double circle of wire?" asked lampert.

"the wire will stop only an animal in control of its motion when it makes contact. if a felodon were to spring from a little distance, it might not like the wire—but it could hardly stop until it reached the ground, and there should then be a similar barrier ahead of it."

"we could use a lethal voltage."

"even if you want to take the risk—what is lethal to a felodon will be equally so to a man—you'll have the insulation problem. there's always a darned good chance of rain before morning, and—"

"we might as well stay inside, then. we have the electric equipment, but it will take quite a while to set it up; and it hardly seems worth the trouble for a one-night stand. as you say, it will be a little crowded here. but we've all slept under worse conditions. would anyone rather set up the fence?"

there was no answer to this question. at lampert's direction a meal was served and eaten. then the scientists settled down for the night, some to sleep at once, others to review plans or recheck equipment. mitsuitei occupied himself with making careful measurements of the photographs he had been given; he was the last asleep....

scores of miles to the southwest, the felodon reached the river. it was no longer on the coast; some time since it had swerved inland. a casual compass check would have revealed that it was still heading straight for the now grounded helicopter. even mclaughlin could not have told what led the creature on, familiar as he was with the animals of viridis; but no one who had watched the thing since the flying machine had passed could have doubted its goal. actually, it was now on the same bank of the river as the helicopter; but whatever guided it pointed across the great stream.

without hesitating, the amphibid plunged into the water.