Ch. XXXIII.] PORPHYPJTIC GRAXITE. 707 



a very moderate depth. Still less does the point to which the ma- 

 terials of granite or lava must be cooled down before they crystallize or 

 consolidate afford any test of the degree of heat which the same ma- 

 terials acquired before they could be made to form lakes and seas of 

 molten rock in the interior of the earth's crust 



To what extent some of the metamorphic rocks containing the 

 same minerals as the granites may have been formed by hydro- 

 thermal action without the intervention of intense heat comparable 

 to that brought into play in a volcanic eruption, will be considered 

 when we treat of the metamorphic rocks, in the thirty-fifth chapter, 

 p. 736. 



Poiyhyrltlc Granite. — This name has been sometimes given to that 

 variety in which large crystals of common felspar, sometimes more than 

 3 inches in length, are scattered through an ordinary base of granite. 

 An example of this texture may be seen in the granite of the Land's End, 

 in Cornwall (fig. 737). The two larger prismatic crystals in this draw- 

 Fig. 737. 



Porphyritic granite. Land's End, Cornwall. 



ing represent felspar, smaller crystals of which are also seen, similar 

 in form, scattered through the base. In this base also appear black 

 specks of mica, the crystals of which have a more or less perfect 

 hexagonal outline. The remainder of the mass is quartz, the trans- 

 lucency of which is strongly contrasted to the opaqueness of the white 

 felspar and black mica. But neither the transparency of the quartz 

 nor the silvery lustre of the mica can be expressed in the engraving. 



The uniform mineral character of large masses of granite seems to 

 indicate .that large quantities of the component elements were thor- 

 oughly mixed up together, and then crystallized under precisely simi- 

 lar conditions. There are, however, many accidental, or " occasional," 

 minerals, as they are termed, which belong to granite. Among these, 

 black schorl or tourmaline, actinolite, zircon, garnet, and fluor spar 

 are not uncommon ; but they are too sparingly dispersed to modify 

 the general aspect of the rock. They show, nevertheless, that the in- 

 gredients were not everywhere exactly the same ; aud a still greater 

 variation may be traced in the ever-varying proportions of the felspar, 

 quartz, and mica. 



