172 On the Granite formation 



tion, presents a scalar appearance in steps sloping down- 

 wards, sometimes three feet high. 



With precaution, the plates separated are sometimes of 

 considerable length and width. They appear to have a 

 tendency to cleavage across the length, but not a decided 

 one in any other way. 



No appearance of seams or any natural separation into 

 lamina is ever visible in the granite, and the surface of the 

 separated lamina presents not the slightest visible difference 

 from the mode of aggregation of any other part of the mass. 



The masses of granite which have a prismatic form, or 

 which occur in crags, do not cleave by the action of fire, 

 and though they yield square bloeks to a series of chissels 

 applied in a row, yet I have not observed that they cleave 

 better in one direction than another ; altogether blocks gene- 

 rally have one face parallel to the stripes, and one at right 

 angles to them. The euritic granite has no cleavage what- 

 ever, rending in a most irregular manner, and is therefore 

 never used for building, nor any other purpose. 



Veins embedded. 



The masses of granite are often intersected by straight 

 wall-sided veins of a large grained kind of granite ; the 

 crystals being sometimes two or three inches across, and 

 these frequently contain packets of dark coloured mica, 

 formed of thin leaves, sometimes three inches across, which 

 are embedded in and cut into the substance either of the 

 crystals of felspar or quartz. These veins are generally 

 from six inches to a foot in width, and they cut sheer 

 through the mass, without apparently deviating in their 

 course, and in this they answer the definition of dykes. 

 Veins which cross their course are sometimes shifted; but 

 on carefully examining the sides of the vein, it will be 

 seen, that there is a perfect, but rapid graduation between the 

 vein and the mass. Occasionally I have observed that 



