122 SNAKE RIVER PLAINS OF IDAHO. [bull. 199. 
would not be expected if one movement was responsible for each class 
of deformations. 
The broad gentle depression of the strata beneath the Snake River 
Plain, previously referred to as the Bruneau artesian basin, involved 
several sheets of lava, which were gentty bent in common with the 
associated beds of lacustral and perhaps in part river sediments. 
Further attention will be given this broad syncline in the second part 
of this paper. 
ROCK DISINTEGRATION AND DECAY. 
Under the climatic conditions now prevailing, which no doubt with 
minor variations have been in continuance for a long period of time, 
rock decay is progressing but slowly, while rock disintegration, through 
the action of changes of temperature, freezing of water, etc. , is much 
more rapid. Mountains of quartzite are practically buried beneath 
their own debris, the fragments showing but little change from the 
unaltered parent rock. Mountains of granite are still more deeply 
sheathed with fragments of their own rock, but in this case much 
chemical alteration is manifest. The surfaces of basaltic lava sheets 
long exposed are covered with loose, rounded stones, usually of a rich- 
brown color, to a depth in most instances of not more than a foot 
or two. Between the weathered stones there is in certain regions, 
but not generally, a soft, rich- brown soil that has resulted from the 
decay and disintegration of the basalt itself, together with line dust 
deposited by the wind. The amount of decay, however, that even the 
oldest sheets of basalt now exposed have undergone is but small. 
Many old lava sheets, owing in part, perhaps, to their having been 
covered by protecting la}^ers now removed, still retain their charac- 
teristic surface markings, such as corrugations, pressure ridges, etc., 
in what may be termed a legible condition. While such markings are 
perhaps not an infallible sign that they were formed at the actual 
surface of a lava flow as already noted, their presence over large 
areas must be taken as evidence that such is the case. Perhaps the 
most conspicuous and general change that the exposed crags and riin 
rocks of basalt have experienced is due to the action of wind-carried 
dust and sand. The rocks usually have a smoothed appearance, 
approaching a polish, and their surfaces are characteristically pitted 
with small, somewhat shell-like hollows, or shallow depressions, with 
smooth interiors. The depressions are probably due in part to the 
flaking of the rocks on account of sudden changes of temperature, but 
the polishing is to be ascribed to the friction of wind-blown particles. i 
These surface features, however, are never so conspicuous or well 
executed, one may say, as many well-known examples where quartzite, 
fine-grained limestone, etc., have been long exposed to the drifting 
sands of deserts. 
