Joly — The Penological Examination of Paving- Sets. 75 



is removed iu these parts, leaving- cavities up to 2 or 3 millimetres 

 in diameter, containing a rusty and scaly-looking residue. 



The Red and Grey Caernarvon Granite. 



The next sets we have to consider are derived from a reddish 

 and a grey granite from Caernarvon. They also are described as 

 " Slippery." They are obviously in a much more slippery state 

 than the foregoing granite. 



Both these are fine-textured granites. In both a pink felspar 

 occurs (orthoclase), but more abundantly in the red granite. A 

 greenish black mineral mottles the surface of each, the nature of 

 which is hardly determinable by the unaided eye. The specific 

 gravity of the red granite is 2612 ; of the grey granite, 2-642. 



Under the microscope, the rock is seen to be in each case of 

 coarser structure than any we have yet examined. A large felspar 

 (orthoclase) exists in botli rocks very abundantly, and in great 

 part densely clouded, along with crystals of small size, also much 

 clouded. The form is columnar, and the idiomorphism well- 

 marked, the crystals being saved from mutual interference by a 

 small quantity — perhaps 10 per cent. — of interstitial quartz. The 

 felspar is mostly orthoclase : some plagioclase is present, no 

 microcline. In spite of the dense clouding with alteration- 

 products, the action on polarized light is in nearly all cases pre- 

 served. We thus regard the felspars, both large and small, as 

 hard and resistant material. Adding the interstitial quartz, we 

 find that the great bulk of the rock — not less than 95 per cent. — is 

 composed of well-knit and very durable minerals. The quartz in 

 parts is pegmatitic with felspar, and in parts again is ophitic in 

 its relations with the felspar. A certain very small amount of the 

 quartz is hypidiomorphic. 



Coloured constituents make up only a small part of the rock, 

 and consist for the most part of a chloritic mineral, probably an 

 alteration-product of mica. This mineral is pale green, shows 

 some cleavage, but often is quite irregular in form, and exhibits a 

 plumose extinction in steely-blue colours. A very little horn- 

 blende is present. A black ore — probably magnetite — also occurs, 

 but sparingly. In the grey granite set, remains of original 

 biotite is found. A little apatite is present along with the 

 alteration-products of the biotite. 



