314 



of the porphyritic felspars when these are present) show good form. The 

 two micas are often seen to be intergrown in parallel lamellae. The striated 

 felspar appears to be generally idiomorphic with respect to quartz and 

 orthoclase ; sometimes, however, it occurs as patches in the orthoclase. 

 The latter mineral is generally devoid of optical uniformity. Different 

 portions of one and the same individual extinguish in slightly different 

 azimuths. Sometimes the different portions give a striped character to 

 the appearance of the section under crossed nicols. Typical microcline 

 appears to be rare, if not altogether absent. 



Quartz generally occurs in the form of grains and granulitic aggre- 

 gates ; occasionally it shows traces of crystalline form and when this is 

 the case it is seen to be idiomorphic with respect to the orthoclase. 

 Irregular inclusions containing more or less liquid, and sometimes also a 

 cubic crystal, are almost always present in the quartz. Liquid carbonic 

 acid is recorded by ROSENBUSCH as occurring in the quartz of a granite 

 from Penzance. The following analyses by Mr. PHILLIPS taken in 

 connection with the one already quoted will give an idea of the compo- 

 sition of the normal granite of the West of England. 



I. II. III. 



Si0 2 ... 74-69 ... 74-54 .. 70'65 



A1 2 ... 16-21 ... 14-86 ... 16-16 



Pe 2 ... tr. ... 2-53 ... 1-58 



FeO ... 1-16 ... -23 ... -52 



MnO ... -58 ... tr. ... tr. 



CaO ... -28 ... -29 ... -55 



MgO ... -48 ... tr. ... tr. 



K 2 ... 3-64 ... 3-73 ... 8-66 



Na 2 ... 1-18 ... 3-49 ... -54 



Li 2 ... 10 ... tr 



H 2 ... 1-23 ... -87 ... 1-22 



99-55 100-54 99-83 



Sp. Gr. 2-64 2-66 2-62 



I. Cam Brea Hill, Redruth. II. Botallack. III. Chywoon Morvah. 



So far we have been referring to what may be termed the normal 

 type of the West of England granite. W"e have now to refer to more or 

 less exceptional varieties. Of all the accessory minerals tourmaline is 

 undoubtedly the most important. It occurs as fair-sized crystals, grains, 

 irregular masses or acicular microlites. The microlites often occur in 

 radiating bundles which penetrate the individuals of quartz. The crystals 

 are of the usual nine-sided form ; sometimes, however, they are six-sided. 

 The amount of tourmaline increases towards the margins of the granitic 

 masses. The increase in the amount of tourmaline is accompanied by the 

 disappearance of mica and finally of felspar ; the ultimate result being a 

 rock composed of tourmaline and quartz (schorl-rock). Good examples 

 of schorl-rock occur at Roche, on the boundary of the Hensborough mass, 

 and on the north side of the Land's End mass near Zennor. The central 

 portions of the granite-masses are sometimes entirely free from tourmaline. 



