PRIMARY ROCKS. 



219 



Slate constitute the chief rocks in the primitive 

 formations. Doubtless the most simple classifica- 

 tion of primitive rocks is into unstratijied and strati' 

 fied ; including under the former granite, syenite, 

 porphyry, and greenstone, and perhaps granular lime^ 

 stone, all of which probably owe their origin to ig- 

 neous causes ; w^hile the latter would embrace 

 gneiss, mica slate, argillaceous, talcose, and chlorite 

 slate, quartz rock, &c., which have been produced by 

 the consolidation of the particles resulting from the 

 disintegration of the igneous rocks, and in some in- 

 stances of that of the quartz rock, modified by sub- 

 sequent heat. At any rate, few geologists of the 

 present day, or perhaps none who deserve the 

 name, can be found who w^ill deny a belief in the 

 igneous origin of the unstratified rocks, and who do 

 not regard them as merely varieties of the same 

 melted mixture, whose peculiarities resulted from 

 the modes in which they were cooled and crys- 

 tallized, and included among the stratified rocks. 

 It by no means follows, from this view of the sub- 

 ject, that all the unstratitified rocks are of the same 

 age, as they were doubtless protruded at diff'erent 

 periods. Granite veins have, for example, been 

 found overlying chalk ; basalt has cut through 

 rocks of every formation ; and lava everyday over- 

 spreads diluvium and alluvium. 



Professor Hitchcock speaks of a pseudo-stratifi- 

 cation of granite in Worcester ; but this is doubtless 

 to be attributed to the process of cooling, under pe- 

 culiar circumstances of position, as we see in ba- 

 salt ; it sometimes assuming a columnar form, at 

 other times not. Throughout New-England granite 

 usually occurs in the form of veins and protruding 

 masses, sometimes lying between the strata of 

 other rocks, but rarely, if ever, overlying them. 

 Professor Hitciicock gives numerous instances of 

 such veins in mica slate, hornblende slate, gneiss, mi- 

 caceous limestone, Sic. Such veins abound in the 



