GRANITIC ROCKS. 



The Family of Quartz-orthoclase Rod's. 



Typical granitic rocks are all perfectly crystalline. The felspar 

 crystals all touch each other without any intervening nncrystalline 

 material ; they may be minute when cooled rapidly, or some inches in 

 length if cooled slowly, when the rock is said to be porphyritic. As a 

 rule felspar is the only mineral in which the crystals are large. Many 

 of the quartz crystals, as was first discovered by Mr. Sorby, contain 

 cavities with a globule of liquid and a bubble of air ; 1 and on their 

 characters an estimate has been made of the pressure under which 

 granite consolidated, as represented in thousands of feet of overlying 

 rocks which have since been removed. In Cornwall this pressure has 

 been stated at 50,000 feet, in the Grampians at 78,000 feet, while in the 

 Lake Country the pressure is considered to have been less. There are 

 five or six chief varieties of rocks of the granite group which agree in 

 containing quartz, besides many other local varieties, resulting from 

 original differences in the rocks which were thus metamorphosed. 



In this country granite is the only rock of the series that will come 

 under the notice of the physical geologist as forming an important 

 element in scenery. In London, examples of some of the chief 

 varieties may be seen in a worked condition in the following public 

 buildings : Thames Embankment, London Bridge, Waterloo Bridge, 

 Club Houses in Pall Mall, pillars in front of St. Paul's Cathedral, 

 Midland Station and Euston Station. 



Quart. Journ. Geol. Soc., vol. xiv. p. 453. 



