OF THE GREEN EIVEE GEOUP. 131 



the myriad of plants and insects are entombed, are wholly composed of vol- 

 canic sand and ash; 50 feet or more thick, they lie in alternating layers 

 of coarser and finer materials. About half of this, now lying beneath 

 the general surface of the ground, consists of heavily bedded drab shales 

 with a conchoidal fracture, and totally destitute of fossils. The upper 

 half has been eroded and carried away, leaving, however, the fragmentary 

 remains of this great ash deposit clinging to the borders of the basin and 

 surrounding the islands; a more convenient arrangement for the present 

 explorer could not have been devised. That the source of volcanic ashes 

 must have been close at hand seems abundantly proved by the difference 

 in the deposits at the extreme ends of the lake. Not only does the thick- 

 ness of the beds differ at the two points, but it is difficult to bring them 

 into anything beyond the most general concordance. 



"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 valley in which they rest. Great masses of the shales still adhere 

 equally on every side to the rocks against which they are deposited, prov- 

 ing that time alone, and no rude agency, has degraded the ancient flora 

 of the lake." 



The examination of Professor Scudder of the deposits of this lacus- 

 trine basin was principally made in a small hill, from which, perhaps, the 

 largest number of fossils have been taken, lying just south of the house 

 of Mr. Adam Hill and upon his ranch. "Like the other ancient islets of 

 this upland lake it now forms a mesa, or flat-topped hill, about 30 to 50 feet 

 high, perhaps 300 feet long and 80 broad. Around its eastern base are 

 the famous petrified trees, huge, upright trunks, standing as they grew, 

 which are reported to have been 18 to 20 feet high at the advent of the 

 present residents of the region. Piecemeal they have been destroyed by 

 vandal tourists, until now not one of them rises more than 2 or 3 feet 

 above the surface of the ground, and many of them are entirely leveled; 

 but their huge size is attested by the relics, the largest of which can be 

 seen to have been 10 to 15 feet in diameter. These gigantic trees appear 



