PART 1. CHAPTER IX. 



121 



Porphyritic Granite Syenite. 



felspar and quartz, presents broken lines, which have been com- 

 pared to Hebrew characters. 



Porphyritic granite. — This name has been sometimes given 

 to that variety in which large crystals of felspar, sometimes more 

 than an inch in length, are scattered through an ordinary base 

 of granite. An example of this texture may be seen in the gra- 

 nite of the Land's End, in Cornwall. (Fig. 110.) The two 



Fig. 110. 



Porphyritic granite. Land's End, Cornwall. 



larger prismatic crystals in this drawing 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 out- 

 hne. The remainder of the mass is quartz, the translucency of 

 which is strongly contrasted to the opaqueness of the white fel- 

 spar and black mica. But neither this transparency of the 

 quartz, nor the silvery lustre of the mica, can be expressed in 

 the enorravinor. 



o _ o 



The uniform mineral character of large masses of granite 

 seems to indicate that large quantities of the component elements 

 were thoroughly mixed up together, and then crystallized under 

 precisely similar conditions. There are, however, many acci- 

 dental, or " occasional," minerals, as they are termed, which be- 

 long to granite. Among these black schorl or tourmaline, acti- 

 nolite, 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 ingredients were 

 not every where exactly the same ; and a still greater variation 

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

 quartz, and mica. 



Syenite. — When hornblende is the substitute for mica, which 

 is very commonly the case, the rock becomes Syenite : so called 

 from the celebrated ancient quarries of Syene in Egypt. It has 

 all the appearance of ordinary granite, except when mineralogi- 



