310 



representatives are common. In the micro-granites and granophyres the 

 double-refraction, due to the fact that the ground-mass is composed of 

 the definite minerals, quartz and felspar, is probably in all cases original. 

 In the felsophyres it may be original or secondary. At the present 

 time we unfortunately possess no criterion by which we can decide as to 

 its true character in the absence of structures which certainly indicate the 

 original presence of glass. Perlitic felsites are, however, very common, 

 and in all such cases we may safely infer that the original rock was a 

 perlitic obsidian or pitchstone. In the majority of cases the perlitic 

 felsites are crypto-crystalline. The double-refracting portions are so small 

 and overlap so frequently as to make it impossible to state anything 

 definite with regard to them, even in the thinnest sections. It sometimes 

 happens, however, that the felsite appears to be micro-crystalline. In this 

 case the boundaries of the individual grains are never so well-defined as 

 in the micro-granites, and a differentiation of the grains into the two 

 minerals quartz and felspar is impossible. Another and a somewhat 

 special type of devitrification is the spheruhtic. Mr. RUTLEY has shown 

 that a perlitic felsite from the Long Sleddale Valley, in the Lake District, 

 is largely composed of minute spherulites giving a well-defined black cross 

 and bearing such a relation to the perlitic cracks as to leave no doubt 

 whatever that they are of secondary origin. These spherulites are positive. 

 What may be termed the micro-crystalline in contradistinction to the 

 common or crypto-crystalline type of devitrification occurs also in the 

 same rock (see Fig. 2, Plate XXXVIII.). 



DESCRIPTION OF THE ROCKS. 



Granites occur as a rule in large intrusive masses. They are distinctly 

 and often coarsely crystalline in the central portions of the mass, but they 

 may become finely crystalline at the margins and in the apophyses. 

 Granites may in fact be seen to pass gradually into micro-granites, 

 granophyres and felsophyres. They vary considerably in colour and 

 composition, but there is generally a close resemblance between the granites 

 of one locality and of one geological epoch. A comparative study of 

 British granites with a view to ascertain the relative importance of the 

 different points of resemblance and difference has not as yet been made. 

 In reviewing what is known as to the granites of different localities some 

 information will be given on these points, but much work will have to be 

 done before our knowledge becomes at all satisfactory. 



A very interesting feature of granites is the frequent occurrence in them 

 of nodules and patches which differ in composition from the main mass of 

 the rock. We are indebted to Mr. J. A. PHILLIPS^) for most valuable 

 information as to the nature of these patches. Sometimes they undoubt- 

 edly consist of fragments of foreign rocks, more or less altered and 

 impregnated with granitic materials; at other times they are composed 

 of the same minerals as the granite associated in different proportions. In 



(1) Q.J.G.S., Vol, XXXVI. (1880), p. 1, 



