LITHOID TUFA. 



191 



formed terraces and embankments, is cemented by it into a compact con- 

 glomerate. 



Lithoid tufa is found nearly everywhere throughout the valleys for- 

 merly occupied by Lake Lahontan, where the conditions for its deposition 

 and preservation were favorable. In vertical range it occurs from the lowest 

 part of the basin now open to inspection, up to a horizon about thirty feet 

 below the highest of the ancient beaches. The broad wave-cut shelf, which, 

 so far as can be determined, is the upper limit of this variety, we have called 

 the Lithoid terrace (see pagelOl). At its upper limit this tufa is seldom more 

 than eight or ten inches in thickness, as it remains to-day after a long period 

 of weathering; but lower in tlie basin it attains a thickness often or twelve 

 feet; what its maximum development maybe is difficult to determine, as its 

 base is nearly always concealed by lacustral deposits or later formed tufas. 



The surface of the lithoid tufa when exposed by the removal of the 

 sheathing of thinolite crystals that usually covers it below the horizon of 





33enaiiiia tuu 



^y^-W-1 -4Z7zir£rAZO 



Fig. 27. — Section showing cnrrent-bedderl gravels between lithoid and thinolite tufa. 



the thinolite terrace, sometimes shows the. effect of weathering which took 

 place previous to the deposition of the second formed variety. In places 

 the two layers are separated by a deposit of pebbles two or three feet in 

 thickness, united bv a calcareous cement, or bv current-bedded gravels, as 

 shown in the following drawing of a section exjiosed at the eastern base of 

 the Marble Buttes. In some instances stones and pebbles were carried 



