44 W. P. Blake — Mineralogical Notes. 



These beds are doubtless remnants of a much more extended 

 deposit which occupied a local lake-like depression, or basin r 

 probably at the close of the great volcanic era during which 

 most of the mountain valleys of central Arizona were tilled up 

 by sediments and then overlaid by successive streams of lava. 

 Sedimentary beds of volcanic origin remain throughout the 

 Yerde valley and its chief tributaries, and in the region of 

 Camp Yerde are deeply eroded, but rest on the uneven floor of 

 ancient pre-Silurian slates standing on edge. High above the 

 deposits of the valley, vertical cliffs of hard lava mark the 

 edges of extended mesas of malpais, under which all the other 

 formations are hidden and protected. But the excavations in 

 the banks of the sulphate of soda are insignificant in compari- 

 son with the magnitude of the beds, and have failed to show, 

 conclusively, any bottom or top, or to reveal the true relations 

 of the beds to the surrounding formations. Whether or not 

 they are members of the volcanic series or of a later and more 

 local origin is yet uncertain. 



Thenardite. — This salt constitutes the bulk of the deposits. 

 It is a coarsely crystalline mass, so compact and firm that it can 

 be broken out only by drilling and blasting with powder. It 

 varies in its purity. Some portions are more or less contami- 

 nated with a greenish colored clay, but it is obtained also in 

 large masses nearly colorless and transparent, with a slight yel- 

 lowish tint, but seldom showing crystalline forms. 



Mirabilite. — The hydrous sulphate of soda occurs in close 

 association with the thenardite and appears to penetrate its 

 mass in veins, but may prove to be an overlying bed. It is this 

 species which, by its rapid effloresence when exposed to the 

 air, covers the whole deposit with a white powder and a thick 

 crust through which the quarrymen must cut before they reach 

 the solid banks of the anhydrous sulphate. 



Halite. — Rock salt in beautifully transparent masses is spar- 

 ingly disseminated in portions of the great beds. These crys- 

 talline masses, so far as observed, do not exceed an inch or two 

 in thickness and no evidence of the existence of any separate 

 workable beds could be seen. It is irregularly disseminated in 

 the sulphate. Some masses exhibit beautiful blue tints, of 

 color, like those seen in the salt of the Tyrol and of Stassf urt. 

 Good fragments for optical and thermal experiments could- be 

 obtained here. 



Glauberite. — This anhydrous sulphate of lime and soda is an 

 interesting associate of the other species. It occurs chiefly 

 near what appears to be the base of the deposits, in a compact 

 green clay. It is in clear, transparent, colorless crystals, gen- 

 erally in thin rhombs, lozenge shaped, with the plane angles of 

 80° and 100°, and from half an inch to an inch or more broad 



