52 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA. 



Size of the lake. The quantity of sediment which accumulated in this lake 

 shows that it was deep, and if it had a proportionate areal extent it must have been 

 a very important geographic feature, of which only a very small part was included 

 in the area mapped. 



Origin of lak<- Ixmht. The lake came into existence at the close of the most 

 active period of the Tonopah rhyolite-dacite eruptions. These lavas, as well as those 

 of the preceding Fraction dacite-breccia eruptions, were poured out on a land sur- 

 face. The formation of the lake was due to a depression of the crust, forming an 

 inclosed basin, or to a climatic change with increased rainfall, or to both com- 

 bined. It is at least certain that there was such an inclosed basin, and while 

 it may have been due to unknown causes, a hypothesis to account for it is 

 suggested. 



The extensive, active, and long-continued dacitic eruptions, which are attested 

 by the Heller dacite, the Fraction dacite breccia, and the Tonopah glassy rhyolite- 

 dacite not only poured out or showered upon the surface a great bulk of lava, 

 but emitted an enormous volume of gas and steam, which mingled with the 

 atmosphere. At the close of the active eruptions there ensued a period of com- 

 parative rest, as is indicated by the presence of fine-grained and undisturbed 

 white tuffs, which were deposited for the most part slowly. As the incompletely 

 occupied spaces left by the violent eruptions were filled the crust subsided of 

 its own weight and the basin was formed. That such collapse occurs around 

 centers of volcanism, consequent on the relief obtained by outbreaks, has been 

 proved by European geologists. 



Sir Archibald Geikie. in his study of the ancient volcanic rocks of Great 

 Britain, refers to the plateau of Antrim in the north of Ireland, as follows: 



. . . Hence the original area over which the iron ore and its accompanying 

 tuffs and clays were laid down can hardly have been less than 1,000 square miles. 

 This extensive tract was evidently the site of a lake during the volcanic period, 

 formed by a subsidence of the floor of the lower basalts. . . . For a long time 

 quiet sedimentation went on in this lake, the only sign of volcanic energy during 

 that time being the dust and stones that were thrown out and fell over the water 

 basin or were washed into it by rains from the cones of the lava slopes around. 



It may here be remarked that the tendency to subsidence in the Antrim plateau 

 seems to have characterized this region since an early part of the volcanic period. 

 The lake in which the deposits now described accumulated was entirely effaced and 

 overspread by the thick group of upper basalts. Hut long after the eruptions had 

 ceased a renewed sinking of the ground gave rise to the sheet of water which now 

 forms Lough Neagh." 



Lough Neagh, which occupies the deepest part of this hollow and covers about 

 one-eighth of the whole area of subsidence, is the largest sheet of fresh water in the 

 British Isles.* 



A m-i.-nt Vulcanoeo of Great Britain, vol. '2. p. 21*. ' Op. clt., p. 1 1.-. 



