408 S. ALLPOKT ON THE METAMORPHTC ROCKS 



many places. At Mousehole, St. Michael's Mount, and Cape Corn- 

 wall, where good junctions are exposed on the shore, there is not 

 the slightest indication of a gradation from one to the other ; the 

 mass of granite cuts sharply through the slates, and has thrown out 

 numerous veins, both large and small, which have penetrated them 

 in various directions. In all such cases the slates have evidently been 

 greatly altered along the line of junction ; and fragments of them 

 have not unfrequently been torn off, and are now enclosed in the 

 granite. There can be no question, therefore, that the granite here 

 presents all the characters of an intrusive igneous rock ; and it will 

 presently be seen that its action on the surrounding strata is chiefly 

 characterized by the development in them of some of the minerals 

 which constitute its own mass, and that occasionally the original 

 lamination of the fine sedimentary matter has been replaced by a 

 very distinct foliated texture. It should be stated, however, that a 

 decided foliation is restricted to the immediate vicinity of the 

 granite. 



Microscopical Examination. 



I. Metamorphic Rocks of Sedimentary Origin. 



Slates in contact with Granite. 



Hand-specimens showing the junction of the two rocks may be 

 obtained with a little trouble ; and, as was observed in the case of the 

 larger masses, the altered slates never pass gradually into granite, 

 but present a distinct line of separation when examined by the naked 

 eye or pocket-lens. This is well seen in PI. XXIII. fig. 1, drawn 

 half the natural size from a polished slab. The dark streaks on 

 each side of the granite-vein represent folia composed of tourmaline 

 with a little mica. 



Tourmaline- Schist. — Under the microscope a thin slice exhibits, 

 however, a kind of junction rather difficult to describe in words. 

 Both rocks then appear to be so intimately associated together as to 

 form one continuous siliceous mass ; they are, in fact, so completely 

 blended that the line of junction can only be recognized by the pre- 

 sence of innumerable grains and crystals of mica or tourmaline 

 which have been developed in the slaty portion of the slice, while 

 the opposite half presents the usual crystalline granular texture of 

 granite. This is the appearance presented when the crystals of 

 mica and tourmaline are moderately large and not too numerous ; 

 when, however, they occur in very small grains, and thickly crowded 

 together, the line of junction becomes far more sharply defined. 

 PL XXIII. fig. 2 represents a junction with slate from Mouse- 

 hole, as seen in a thin slice cut from the same specimen as fig. 1. 

 The granite has here cut through the slate obliquely to the plane of 

 bedding ; and this coincides in direction with certain lines of folia- 

 tion, in which small grains of mica and tourmaline are arranged in 

 irregular streaks alternating with narrow bands and lenticular 

 patches of quartz. The folia containing the larger grains consist 



