46 Diorites and Granites of Swift's Greek, 



In addition to the quartz and felspars, hornblende and 

 magnesia mica occur either singly or together. In the 

 gneissic rocks of this class (plagioclase gneiss) of Riley's 

 Creek, mica predominates almost entirely to the exclusion 

 of hornblende. But when microcline represents the tri- 

 clinic felspars, I find both. In addition, the accessory con- 

 stituents are apatite, titanite, magnetite, and pyrite. These 

 rocks are, therefore, quartz diorite and quartz mica diorite; 

 and they are connected by those rocks which contain both 

 hornblende and mica. 



It must be noted that a rock composed of quartz, a potash 

 felspar, and hornblende or mica, would be a granite ; but as 

 the distinctive feature of the quartz diorites is that their 

 felspathic constituent is triclinic, I have considered those 

 granitic rocks in which microcline is the felspar to be a 

 peculiar form of quartz diorite, to which class they are in 

 position more nearly allied than to the adjoining granites. 



Diorites. — The true diorites — that is, rocks composed 

 of a triclinic felspar and hornblende, or of those 

 minerals together with mica — are found at Riley's Creek, 

 Sheep Station Creek Gap, Upper Swift's Creek, and 

 Eureka, and are all, so far as I can ascertain, subsequent 

 in age to their more acid allies. The two constituents, 

 plagioclase and hornblende or mica, vary much in their 

 relative proportions ; so that on the one hand there 

 results a crystalline granular felspar rock with a little 

 hornblende or mica, and on the other a crystalline or crystal- 

 line granular hornblende rock with a little triclinic felspar ; 

 but so far as I have observed, mica does not anywhere, like 

 amphibole, preponderate to the exclusion of the other con- 

 stituents. 



It seems that when the felspar predominates it is princi- 

 pally anorthite with a little and very subordinate soda 

 lime felspar (albite, or oligoclase), and the associated mag- 

 nesia mica a phlogopite ; when the hornblende predominates, 

 the felspars are probably albite, oligoclase, labradorite, or 

 anorthite, and in extreme cases are almost invariably only 

 present as imperfect crystals or granules in the amphibole. 

 These rocks would, therefore, be varieties of diorites, varying 

 in character from a compound nearly approaching amyflii- 

 oolite to one which on account of its felspar being anorthite 

 would be allied to corsite. 



Amphibole-gabbro. — I have found adjoining those 

 varieties of diorite in which hornblende predominates 



