GEOLOGICAL SUEVEY OF THE TERRITOEIES. 23 



noted. The valley is about ten miles wide and is entirely occupied witli 

 the Pliocene beds from side to side. The terraces underlaid by this 

 deposit are a marked feature, and rise 300 feet above the creek, the 

 middle one 150 feet and the lowest 50 feet. The hills on the west side 

 are lower and less rugged, rising 400 to 1,000 feet above the valley j but 

 on the east side they are more formidable, 1,500 to 1,800 feet in height. 

 The surface outlines are quite rounded and smooth by weathering, so that 

 the strata are not well defined. Marsh Valley, which is about five miles 

 in length, is like a meadow covered with tall, thick grass. Soon after 

 l)assing the divide, a small stream commences running northward toward 

 the Port Neuf, and on either side are wide, swampy, or springy belts, 

 contributing springs at every step, and in a distance of ten miles it be- 

 comes a good-sized river. The luxuriance of the vegetation is a marked 

 feature. The entire channel was filled vnth several species of water- 

 plants, Fotamogeton^ EanunculuSj Brasenia, MyriopJiylhim^ and many 

 others. As a necessary result, the fresh- water molluscous life was most 

 abundant, Flanorhis, Limnea, Fhysciy &c. 



About ten miles north of Carpenter's Station we come to the southern 

 border of the great basaltic overflow in the valley of Port Neuf and Snake 

 Eiver, for I am now convinced that this comparatively modern eruption 

 of igneous material covered an immense area of country, and might be 

 called the basin of a wide, extended lake of igneous material, of which 

 the Snake- River Basin was the center. Whether the melted material 

 flowed up the valleys of the streams that empty into the Snake Eiver, or 

 issued from fissures extending up these valleys and overflowing them from 

 side to side, it is difficult to determine. The latter explanation is most 

 probably the true one, judging from the uniformity in thickness and extent 

 of this vast sheet of lava. The elevation of our camp on the south border 

 of the lava basin inPort I^euf Valley is 4,625.5 feet, and this seems to have 

 been the height to which it reached in its overflow. The little streams 

 have cut new channels directly through the lava flooring, and thus excellent 

 sections of it may be studied. As a rule the streams flow along deep muddy 

 channels, with boggy border and abrupt sides obstructing and even ren- 

 dering the fording of them dangerous : and on either side, varying in dis- 

 tance from a few yards to a half a mile, is a vertical wall of basalt, 

 which, in the distance, has a partially columnar appearance. The basalt 

 fractures into vertical masses that have an obscure five or six sided form. 

 Sometimes these walls are so steep 'and uniform for miles that they can- 

 not be scaled, and some broken-down, eroded portion must be sought for 

 before the traveler can escape from the marshy channel of the streams 

 to the table-like plateau above. The lower portion of this lava floor is 

 very comiDact and massive, but the top part is more or less vesicular. 

 There is very little, if any, of the usual spongy lava ; it is all very heav}", 

 even though full of cavities. It effervesces freely, showing the presence 

 of lime in considerable quantities. The illustrations of exfoliation are 

 abundant everywhere. Sometimes quite thick beds show an exposed 

 surface of rounded masses, decomposing in concentric layers as if it 

 was an aggregation of large concretions. The disintegration of these 

 igneous rocks is mostly accomplished through the process of exfoliation. 

 The general appearance of this table-shaped belt of basalt contrasts 

 strangely with the ranges of hills on either side. On the east side of the 

 valley the foot-hills are quite irregular, high, and covered with drift. On 

 the west side they slope rather gently down to the river, deeply cut 

 here and there by ravines. Thcsui)eriicial deposits extend high up, 500' 

 feet or more above the bed of the river, lapping smoothly on the basis 

 rocks. The white Pliocene sandstones are exposed at one locality not 



