144 MINERALOGY AND LITHOLOGY. 



Granitic. 

 Granite. 

 Sienite, 



Crystalline Schists. 

 Gneiss. 

 Mica Schist. 

 Argillitic Mica Schist. 

 Quartz Schist. 



Greenstones. 

 Metamorphic Diorite.* 

 AmphiboHte. 

 Chlorite Schist. 



Half Crystalline Rocks. 



Clay Slate. 

 Quartz Schist. 



Minerals as rocks have been treated of in the preceding chapter. 

 Fragmental rocks have been discussed in the part upon Surface 

 Geology, and are hence very briefly treated here. 



BASIC ERUPTIVE ROCKS. 



The investigations which thus far have been made upon our basic 

 eruptive rocks leave much to be desired in detailed knowledge of their 

 nature and composition. The dark-colored rocks which intrude them- 

 selves, here abundantly, there sparingly, through our crystalline strata, 

 have in some cases been correctly identified from their coarse-grained 

 and porphyritic varieties ; but, when examined with the microscope, rocks 

 which are apparently alike are found to be so different, — so many unsus- 

 pected ingredients are found to be present, so many suggestive structu- 

 ral effects arc seen, — that it becomes plain that such study is necessary 



Moreover, in the present confusion (especially among writers in the English language), while every writer uses 

 his own nomenclature, and impresses on each name his own signification, the adopted method will most certainly 

 convey the idea that is intended. 



No new names have been introduced, though I am certain that, according to precedent, material is at hand. 

 This feature of my report I hope will be commended. 



* This rock is mostly massive. I think it will be found satisfactorily explained, however, why it is considered 

 in this connection. 



