No. 3.] MACFARLANE— ON CRYSTALLINE ROCKS. 267 



case in fine grained compound rocks. By giving prominence to 

 each of these non-felsputhic minerals and placing their nr.mes on 

 the horizontal lines of our table, it becomes possible to shew at a 

 ghmce the rocks which they form with the felspathic minerals 

 named at the heads of the vertical columns, and the manner in 

 which, by gradually replacing each other, they form the different 

 species of original rocks. Thus it will be observed that among 

 the schistose rocks the most basic is diabase schist ; that the latter 

 becomes diorite schist when hornblende replaces pyroxene ; that 

 the diorite schist, as its oligoclase is replaced by orthoclase. becomes 

 syenite schist, and, as quartz makes its appearcnce f.nd increases, 

 syenitic gneiss is produced. At the next step in a silicic 

 direction, mic i replaces the hornblende, producing common gneiss, 

 then when the mica disappears, granulite results. If, instead of 

 the mic-i, the orthoclase disappears, mica schist is developed, and 

 when from the latter rock the mica in greater part is withdrawn, 

 it becomes quartz schist. The other varieties of texture, such as 

 the porphyritic and trachytic, each exhibit a similar series of 

 transitions, the most fully developed being the grunular order. 

 In the latter it becomes possible, by meansof the peculiar arrange- 

 ment of our table, to shew the mineralogical nature of each of 

 the species of the complicated family of the greenstones. Diorite, 

 gabbro, hyperyte, diabase and protobastyte rock are shewn to be 

 respectively characterised by hornblende, diallage, hypersthene, 

 pyroxene and enstatitc in combination with various felspars. The 

 great majority of original rocks contain some variety of felspar, 

 but there are a few species in which that mineral is absent and 

 which are called non-felspathic rocks. In order as far as possible 

 to shew these also in our table, two columns have been added to 

 it, one at each side. The right hand one shews the silicic, and 

 the left hand the basic rocks void of felspar. 



VI. — ACCESSORIAL CONSTITUENTS. 



Besides the minerals mentioned in the foregoing chapter as the 

 essential constituents of crystalline rocks, there are others of less 

 frequent and only accidental occurrence, which have been called 

 by German lithologists the accessorial constituents. Among these 

 such minerals are not included as are only found in the veins, 

 cavities, or even joints enclosed in rocks. Only those which are 

 found in intimate mechanical union with the essential constituents 

 in the body of the rock itself are regarded as accessorial consti- 



