101 



the presence of crystals of orthoclase, and sometimes of crystals 

 or grains of quartz imbedded in the finely granular or impal- 

 pable paste. These felsites and felsite-porphyries are, in very 

 many cases at least, stratified or indigenous rocks, and they are 

 sometimes found associated with granular aggregates of dif- 

 ferent degrees of coarseness, which show a transition from true 

 felsites into granitic gneisses." 



DIOEITE. 



It is intended to include here only clearly eruptive rocks, 

 using the term eruptive in a general sense that wUl embrace all 

 rocks that have been fluent, whether extravasated or not. 

 There are stratified diorites in this formation, but they are in- 

 separable from the other members of the stratified or gneissic 

 series, and may be most conveniently described in that relation ; 

 although, as will appear farther on, the chief distinction be- 

 tween the dioritic group and the stratified group consists in the 

 greater disturbance and alteration which the former has ex- 

 perienced, the' two having been originally essentially the same. 



Even as thus limited the dioritic group includes a considerable 

 variety of rocks as regards both texture and composition ; and 

 diorite, although probably the best lithologic designation that 

 can be employed, is far from being descriptive in every case. 

 As a rule the diorite is fine-grained, sometimes impalpably fine, 

 exhibiting local transitions to felsite, as already explaiaed, and 

 never attaining the degree of coarseness common with the 

 granites . In composition it has a wider range ; and we not 

 only find all gradations from a rock composed almost wholly of 

 feldspar to one in which hornblende is the sole constituent or 

 nearly so ; but each of these, especially the less hornblendic, is 

 prone to pass, by the addition of quartz, into hornblendic 

 granite. In fact, the areas colored as diorite on the map embrace 

 a large amount of fine-grained, hornblendic granite. This is 

 the granite referred to on a preceding page as always associated 

 with diorite, and rarely with the typical granite and petrosilex. 

 It is frequently little more than a quartzose diorite, and the 



