THE ROCKS OF THE WASHOE DISTRICT. 51 



Field habit. — Tlie commonest variety of tlie east-conntr}- diabase is a fine- 

 grained l)lackish-green rock, the most noticeable macroscopical peculiarity of 

 which is its tendency to develop smooth fissure planes. Sometimes these 

 planes are parallel, and of course divide the rock into sheets. In other cases, 

 quite as common, they form all sorts of angles with one another, and divide 

 the rock into polyhedral fragments, almost like large crystals, or into prisms of 

 various angles; but I failed to find any law governing the angular relations. 

 There can be little question that the cleavages of the rock have been 

 developed by the dynamical action which has repeatedly racked the hang- 

 ing wall; but the tendency to jointing and the planes of cleavage may 

 have been involved in the original structure of the rock, for the hammer 

 develops only the imperfectly conchoidal and somewhat rough surfaces, 

 which other fine-grained rocks show when fractured, and not smooth planes 

 Possibly, however, such might result from a slow but irresistable pressure. 

 The coarse-grained diabases show much less of this jointing, but the fract- 

 ure of both presents the same appearance except in regard to scale — a 

 granular surface with frequent larger lath-like plagioclases. In a great 

 proportion of cases the feldspars are pellucid, even when the augite is 

 wholly decomposed; but when the coarser rocks are so far altered that 

 the feldspars become opaque, the rock looks very like diorite, a resem- 

 blance which is greatly increased by the comparative absence of joints. 

 The diabase on the south side of Ophir ravine looks very like a diorite, 

 though here the exposure is so large that the jointing is clearly visible. 

 In many cases under ground it Is little developed, not more so than is fre- 

 quently the case with the diorite. In a few places, as for example the 

 2,700-foot level of the Yellow Jacket, there are limited occurrences of exces- 

 sively fine-grained, closely laminated diabase resembling slate. The diorites 

 and both the andesites show the same phenomenon. 



It will be seen that the andesites behave very difterently in subterra- 

 nean and subaerial decomposition. The behavior of the diabase in this 

 respect cannot be directly conq)ared with the later rocks, because the ex- 

 posure in Ophir ravine is but little aff'ected, and that near the Ward is obscure 

 and almost wholly covered with wash; but the protection of occasional 

 masses of diabase from decomposition by accidental arrangements of fissures 



