354 



ORIGIN OF ERUPTIVE AND PRIMARY ROCKS. 



above described is so inconsiderable, tbat it does not appear un- 

 reasonable to attribute to the heavenly bodies the powe,r of 

 causing it. Still it would be very desirable if mathematicians 

 would devote some attention to the subject. 



The rocks of the granite family have been arranged by Cotta* 

 according to their granular and slaty varieties, as follows: 



ESSENTIAL CONSTITUENTS. 



Feldspar and Hornblende. 

 Feldspar, Quartz, Mica and 1 

 Hornblende. I 



Feldspar, Quartz and Mica. 

 Feldspar, Quartz and Talc. 

 Feldspar, Quartz and Chlorite. 

 Feldspar, Quartz and Graphite. 

 Feldspar, Quartz and Iron Mica. 

 Feldspar, Dichroite and Mica. 

 Dichroite, Mica and Garnet. 

 Feldspar, Eleeolite and Mica. 

 Feldspar, Quartz and Schorl. 

 Quartz, Schorl and Topaz. 

 Oligoclase and Mica. 

 Feldspar and Quartz. 

 Quartz and Mica. 



RESULTING ROCKS. 



GRANULAR. 



Syenite. 

 Granitic Syenite. 



Granite. 

 Protogine. 

 Chloritic Granite. 

 Graphitic Granite. 

 Iron Granite. 

 Dichroite Granite. 

 Dichroite Rock. \ 

 Miascit. \ 



Schorl Granite. 1 

 Topaz rock. 

 Kersantite. 

 Granulite. 

 Greisen. 



Slaty Syenite. 

 Syenitic Gneiss. 



Gneiss. 



Protogine Gneiss. 

 Chloritic Gneiss. 

 Granitic Gneiss. 

 Iron Gneiss. 

 Dichroite Gneiss. 



not known. 



Kersantite. 

 Granulite. 

 Mica slate. 



The sedimentary rocks of the granite family are as follows :f 



Granitic conglomerate. 



Syenitic do. 



Gneiss breccia and gneiss conglomerate. 



Arkose (Feldspathic sandstone). 



These latter rocks, however, bear but little analogy to the 

 tufaceous rocks of later eruptive formations. Instead of being 

 formed and deposited simultaneously with their corresponding 

 massive rocks, they have generally been derived from the abrasion 

 of these, long after their eruption and solidification, and deposited 

 with the rocks of comparatively recent sedimentary formations. 

 There do not seem to exist or have been formed with granitic erup- 

 tions any rocks of a tufaceous character. The obvious inference 

 to be drawn from this circumstance is, that during at least the 

 older granitic eruptions no water or ocean existed on the earth, 



* Gestein slehre, p. 114. 

 t Lehrbuch, I, 702. 



