74 GEOLOGICAL HISTORY OF LAKE LAHONTAN. 



the crater; near the surface of the lake there are two planes of unconform- 

 ability, as well as a number of small faults. In the crater walls on the 

 opposite side of the lake a number of displacements may be seen, as indi- 

 cated in Fig. E, Plate XVII. 



The form of the cones and the nature of the material of whicli they are 

 composed leave no doubt that these are crater-rings, i. e., low cones of erup- 

 tion containing large craters. The evidence sustaining this conclusion is 

 abundant In the stratified beds of yellowish lapilli, which are always an- 

 gular and sometimes as fine as dust, are many fragments of basalt, rhyo- 

 lite, and masses of hardened lake-beds," that are evidently ejected frag- 

 ments that have been dropped from a considerable height to the positions 

 which they now occupy. The strata of lapilli beneath these "bombs" are 

 bent down, as shown in the accompanying sketch (Figs. B, C, and D, Plate 

 XVII) the disturbance being visible for (i or 8 inches below the included 

 rock. The strata of loose cinders covering the inclosed fragments are 

 horizontal and undisturbed. That the cones were not formed during a 

 single eruption, but have a long and complicated history, and are perhaps 

 sublacustrine in their origin, is shown by the alternation of ejected and 

 sedimentary materials in the crater walls. 



From the presence of fossiliferous lacustral clays in the midst of lapfUi, 

 it seems evident that volcanic eruption was interrupted by periods dui-ing 

 which the lake covered the craters. The presence of dendritic tufa in the 

 midst of the section proves that the volcano was active both before and after 

 the dendritic stage of Lake Lahontan. The wall of the larger lake is some- 

 what open on the south side, while the western rim lias been prolonged 

 southward (see Plate XVI) in such a manner as to suggest that the ernpted 

 material was in part removed by currents at the time it was ejected and 

 deposited in the form of an embankment, connecting with the ci'ater rim. 



The hypothesis that the craters were formed by the action of extremely 

 powerful sublacustrine springs, as advanced by King,'^ would not account 

 for the nature of the material forming the crater walls, nor the presence of 



^iTbe rhyolite pebbles and fragments of lacustral sediments thrown out by this volcano were evi- 

 dently derived from the superficial strata Ihrough which it opened a passage. The basalt, on the other 

 hand was erupted in a semi-fused condition and formed slaggy masses on cooling. 



3*11. S. Geological Exploration of the Fortieth Parallel, Vol. 1, p. 512. 



