20 TERTIARY INSECTS OF NORTH AMERICA. 



ment for the present explorer could not have been devised. That the source 

 of the volcanic ashes must have been close at hand seems abundantly 

 proved by the difference in the deposits at the extreme ends of the lake as 

 will be shown in the sections to be given. Not only does the thickness of 

 the beds differ at the two points, but it is difficult to bring them into any- 

 thing beyond the most general concordance. 



There are still other proofs of disturbance. Around one of the granitic- 

 islands in the southern lake basin the shales mentioned were capped by from 

 one and a half to two and a half meters of sedimentary material, reaching 

 nearly to the crown of the hill, the lowest bed of which, a little more than 

 three decimeters thick, formed a regular horizontal stratum of small vol- 

 canic pebbles and sand (A and B of Dr. Wadsworth's note, further on), 

 while the part above is much coarser, resembling a breccia, and is very un- 

 evenly bedded, pitching at every possible angle, seamed, jointed, and 

 weather-worn, curved and twisted, and inclosing pockets of fine laminated 

 shales, also of volcanic ash, in which a few fossils are found (C of Dr. Wads- 

 worth's note). These beds cap the series of regular and evenly stratified 

 shales (D of the same note), and are perhaps synchronous with the disturb- 

 ance which tilted and emptied the basin. The uppermost evenly bedded 

 shales then formed the hard floor of the lake, and these contorted beds the 

 softer, but hardening, and therefore more or less tenacious, deposits on that 

 floor. 



The excavation of the filled-up basin we must presume to be due to 

 the ordinary agencies of atmospheric erosion. The islands in the lower lake 

 take now as then the form of the granitic nucleus; nearly all are long and 

 narrow, but their trend is in every direction both across and along the val- 

 ley in which they rest. Great masses of the shales still adhere equally on 

 every side to the rocks against which they. were deposited, proving that time 

 alone and no rude agency has degraded the ancient floor of the lake. 



The shales in the southern basin dip to the north or northwest at an 

 angle of about two degrees, and according to the contours of the Hayden 

 Survey, the southern end of the ancient lake is now elevated nearly two 

 hundred and fifty meters above the extreme northwestern point. The 

 grrtitiM 1 p;nl of this present slope of the lake border will be found in the 

 southern half, where it can not fail to at once strike the observant eye, the 

 southernmost margin close to the summit of the divide being nearly two 

 hundred meters higher than the margin next the hill by the forks of the road. 



