Ch. VI.] PRIMARY ROCKS. 99 



are well acquainted with this fact, and therefore never at- 

 tempt to break the rock" in a line diagonal thereto ; and 

 geologists are indebted to them for some curious information 

 on this subject, which has been recorded by Mr. Enys. The 

 granite of Penryn is principally composed of two kinds : one 

 is a hard and compact rock, which is extensively worked and 

 shipped for the London market. It runs in parallel ranges, 

 bearing N. E. and S. W., through the other kind, which is 

 softer and of a coarser texture. The hard granite is cloven 

 into quadrangular blocks ; and it has been ascertained by 

 long experience, that the cleavage cannot be effected in every 

 direction, but only with regularity in three, and that each of 

 these require a different degree of force : thus, if the horizontal 

 cleavage demands a power denoted by two, the perpendicular 

 cleavages, crossing each other, will be in the proportion of 

 three and five ; the latter commonly intersects the larger fel- 

 spar crystals, whilst the former is parallel therewith, and may 

 be termed the longitudinal cleavage. Near Penryn the last 

 mentioned line of cleavage runs N. N. W. and S. S. E., varying 

 15 or 20 either way: it generally coincides with one of the 

 vertical natural joints, though in many instances, it does not 

 correspond therewith, but crosses them often at- an angle of 

 30 or 40.* This interesting fact cannot fail ultimately to 

 lead to some important information, as it will direct the atten- 

 tion of geologists to the condition of the granite of other 

 countries in this respect. 



This subject has not altogether escaped notice, as will be 

 seen in the following extracts ; but it does not appear to have 

 suggested the idea that the layers into which granite is 

 divided by such seams having a determinate bearing are 

 similar to the beds of stratified rocks, nor has it given rise to 

 more extended enquiries. 



" The granite of Reville, in the department of La Manche," 

 says De la Beche, "generally resembles that of Dartmoor, 

 containing large porphyritic crystals of felspar. It splits in 

 * Lond. and Edin. Phil. Mag., vol. ii. p. 321. 

 H 2 



