192 GEOLOGICAL HISTORV: OF LAKE LAHONTAN. 



upwards by the action of sub-lacustral springs about which tufa was being 

 deposited, and became inclosed in the calcium carbonate as it was precipi- 

 tated; this occurrence is not to be mistaken, however, for the partings of 

 current-bedded gravel described above, which must have been deposited by 

 waves and currents in shallow water. The weathering of the hthoid tufa 

 is not always apparent, but has been observed in a number of instances, 

 and can onl)^ be interpreted on the suppostion that the first-formed tufa was 

 exposed to subaerial erosion by a lowering of the lake previous to the depo- 

 sition of the thinolite crystals. The surface of the lithoid tufa above the 

 upper hmit of thinolite is weathered to a much greater extent than below 

 that horizon, indicating that its erosion continued throughout the low-water 

 stage during which thinolite was forming. 



The lithoid tufa of the Lahontan basin is identical in structure and 

 general appearance with the greater part of the tufa deposited in Lake 

 Bonneville, and has essentially the same chemical composition. It is also 

 represented on a limited scale in a number of the minor Quaternary lake 

 basins of Utah and Nevada. Similar deposits are now forming in Pyramid, 

 Walker, and Winnemucca lakes, as described in Chapter III. It is evident 

 that this variety of tufa corresponds in all respects with the first calcareous 

 deposit precipitated when ordinary meteoric waters are evaporated. 



THINOLITIC TUFA. 



Succeeding the lithoid tufa in order of deposition, and forming another 

 coating on the sides of the Lahontan basin, is a deposit of interlaced crystals 

 of calcium carbonate, which were called "Thinolite'' by Mr. King.*® In the 

 reports of the U. S. Geological Exploration of the Fortieth Parallel, however, 

 this term is used to designate any of the calcareous deposits of the ancient 

 lake, Avhether crystallized or not. In the present report we restrict the name 

 to the variety of tufa exhibiting a definite crystalline structure, presently 

 to be described. We may remark, in passing, that no crystals having a 

 resemblance to thinolite have been found in the Bonneville basin; nearly all 

 the calcium carbonate there deposited is of the character of lithoid tufa, as 



«« U. S. Geological Exploration of the Fortieth Parallel. Washiugton , 1878, Vol. I, p. 517. 



