194 GEOLOGICAL HISTORY OF LAKE LAHONTAK 



is strongly carved and forms a consijieuous feature in the contour of the 

 island when seen from a distance; on the northern side of the island the 

 thinolite tufa extends about 15 feet above the water-line of the terrace 

 and forms a thin wedge of interlaced crystals included between the lithoid 

 tufa and the heavy deposit of dendritic tufa. The upper limit of the thin- 

 olite pi'obably varies in the several basins in which it occurs, but no accu- 

 rate measurements of differences of level have been made. 



The sheathing of thinolite extends from the lithoid terrace down to the 

 lowest point in the bottom of the basin now open to inspection. Near its 

 upper limit it is from 6 to 8 feet thick, but increases to 10 or 12 feet near 

 the surface of Pyramid Lake. Like the lithoid tufa it was deposited in 

 successive layers, as is shown by its banded structure. Throughout the 

 mass we find definite layers or zones of small and large crystals alternating 

 with each other. Near its outer limit the bands of crystals are divided by 

 narrow layers of sheets of dendritic tufa; indicating that the condition of 

 crystallization were alternately favorable for the production of the crystal- 

 line or the dendritic structure. The circumstances favorable for the for- 

 mation of the latter finally prevailed, and only dendritic tufa was deposited. 



PROFESSOR Dana's crystallographic study of thinolite. 



While carrying on the field study of Lake Lahontan, large quantities 

 of the difi"erent varieties of tnfa were collected, and special attention given 

 to securing as many specimens as could well be desired for illustrating the 

 structure and mode of occurrence of thinolite. A representative portion 

 of this collection, together with similar material from the Mono Lake 

 basin, was placed in the hands of Profs. G. J. Brush and E. S. Dana for 

 the purpose of obtaining an authoritative statement of the mineralogical 

 relations of thinolite. This study was finally undertaken by Professor Dana, 

 whose report forms Bulletin No. 12 of the publications of this Survey. To 

 the student of Lahontan history this report is especially welcome as it clears 

 away the previous hypothesis, advanced by King, to the effect that thino- 

 lite is a pseudomorph after gaylussite. The following description of thin- 

 olite is quoted with slight verbal alterations from Professor Dana's buUe- 



