196 GEOLOGICAL HISTORY OF LAKE LAHOIsTAiSr. 



smaller, they have widely divergent positions, interpenetrating each other, 

 and giving a large mass an open, reticulated appearance. In addition to 

 the elongated crystals, numerous smaller ones, half an inch or an inch in 

 length, make np parts of J:hese masses, projecting from the sides of the 

 larger crystals and forming divergent groups among themselves. The small 

 crystals have generally the form of an acute ])3'ran)id, and are sometimes 

 square in outline, sometimes rhombic; the sides are usually concave, and 

 the edges project sharply. Tlie exterior surface of the larger crystals is 

 rough and open, often with a delicate mossy covering, and the whole crystal 

 is porous throughout, as if eaten out so as to leave only a skeleton behind. 

 Upon a superficial examination no regularity in the structure is evident, but 

 on looking more closely it is seen that the apparently rough and irregular 

 surface is made up of portions of thin plates, each set parallel to the sides 

 of the crystal and ixniformly converging in one direction. Thus when one 

 of the groups of nearly parallel crystals is viewed end on, from one ex- 

 tremity or the other, it is seen that the edges of the plates, irregular as they 

 are in outline, are all presented to view at once, as if each crystal, though 

 prismatic in general outline, were made up of a series of acute skeleton 

 pyramids, hopper-like in form, placed one within another. Still further, 

 when the section produced by the cross-fractui-e of one of these elongated 

 crystals is examined, there is seen, more or less distinctly, a series of appa- 

 rently rectangular ribs forming concentric squares or rectangles, Avith diag- 

 onal ribs joining the opposite angles. 



The specimens in hand from the Needles, Pyramid Lake, correspond 

 closely with those which have been described, though hardly showing the 

 structure so clearly. This is also true of some of those from Anaho Island. 

 The majority from the latter locality, however, are much more firm and 

 compact. Here, too, the crystals are usually elongated, and in a single 

 specimen grouped in nearly parallel position. The edges of the plates are 

 also commonly distinct on the sides, and show the same convergence toward 

 one extremity. The masses, however, instead of being open and porous, 

 and consequently light in the hand, are close, compact, and heavy. Instead 

 of the delicate, open skeleton, with fretted surface, seen in the cross-fracture, 

 the section is nearly solid, and sparkles with the reflection from the cleavage 



