THINOLITIC TUFA. 195 



tin" to which the reader should refer for a, more complete elucidation of 

 the subject. 



In general it may be said that the thinolite collected from different 

 localities, both in the basin of Lake Lahontan and of Mono Lake, while 

 varying widely in external aspect, is yet remarkably uniform in all essen- 

 tial characters. It is thus established beyond question that the original 

 mineral deposited was throughout the same, although, in consequence of 

 the varied conditions to which it has been subjected, the forms resulting 

 from its alteration are A'ery diverse. Thus, in some specimens there is only 

 a delicate skeleton remaining, the whole consisting of thin plates, held to- 

 gther in their parallel position by a slight central frame-work, while in 

 others the whole is as firm and compact as a ciystalline limestone, and be- 

 tween the two extremes many interesting varieties occur. The most impor- 

 tant condition tipon which this difference depends is the varying extent to 

 which a deposition of calcium carbonate has taken place subsequent to the 

 first alteration of the original mineral. 



As has already been stated, the thinolite is most characteristically de- 

 veloped about Pyramid Lake. The writer has had in hand specimens from 

 the Marble Buttes, from the Needles, from Anaho Island, and from the 

 Domes, and, as they illustrate well the different varieties, it will be conven- 

 ient to refer to them by localities, although no special significance is prob- 

 ably to be attached to the particular spot from which the individual speci- 

 mens were collected. 



The delicate, open, porous variety of thinolite is best shown in the 

 specimens from the Marble Buttes, of which illustrations are given in 

 Plates XXXII and in Fig. 1 of Plate XXXIII. The external form of the 

 crystals is roughly that of a rectangular prism, with projecting edges and 

 generally tapering toward the extremities. The color is gray to brown. 

 These crystals are commonly from a quarter of an inch to an inch in diam- 

 eter and up to 8 or 10 inches or more in length. They are generally 

 grouped in a more or less closely parallel position, often compactly, with 

 only ver}^ little interlacing. In other cases, especially when the forms are 



*'Page 15, et seq. — Tbe numbers referring to Plates in this quotation have been changed so as to 

 denote their position in the present volume. 



