part 2] THE SHAP MINOR INTRUSIONS. 121 



element. Alkali-felspars, quartz, and mica are the usual pheno- 

 • erysts, and the rocks include granite-porphyries, quartz-porphyries, 

 and quartz-felsites. 



The specific gravity varies from 2.J4 to 2 (52, and is always less 

 than that of the granite. A study of the dykes from this point of 

 view yields interesting results. Assuming that the specific gravity 

 hears a definite relation to the basicity of a rock, and can thus be 

 used as a means of comparison, then the acid dykes are always 

 more acid than the granite. The highest density prevails near the 

 granite, three dykes quite close giving 2020, 2 - 614, and 2G27 

 respectively. Considering the intrusions at varying distances 

 to the south, there is a gradual decrease in density, strongly 

 marked at first, but with a diminishing rate in the outer area as 

 the rocks are traced outwards from the granite mass. This is 

 -accompanied by a decrease in the porphyritic element, the ground- 

 mass becoming dominant in the outer parts of the area. Where 

 the large orthoclase-phenocrvsts are present the rocks are usually 

 of a greyish colour, striated felspars are prominent, and biotite 

 is common, the abundance of phenocrysts of all kinds (including 

 quartz) sometimes being very marked. Where the orthoclase- 

 phenocrvsts are absent, the rocks are less porphyritic and more 

 compact, a more or less reddish felsitic ground-mass carrying small 

 phenocrysts of quartz, orthoclase, some oligoclase, and generally 

 less biotite than the preceding. Members of these two groups are 

 sometimes found in association ; but. while the former are restricted 

 to the centre of the area, the latter have a wider and more general 

 distribution. The division based on field-evidence is supported by 

 certain differences in microscopic structure. The two groups were 

 intruded at different times, and it is of interest to note that the 

 latter and later group is more acid than the earlier. 



In the orthoporphyritic rocks the large orthoclases attain a 

 length of 2 inches ; thev are commonlv twinned and always more 

 or less corroded, but are never so abundant as in the granite. 

 Traced outwards from the centre of the area, they decrease both in 

 size and in relative importance. Abundant and of maximum size 

 2 miles away, they are only half the size at double the distance, 

 and beyond that no longer form the most conspicuous porphyritic 

 mineral. They are usually pink in colour ; but white and almost 

 colourless varieties also occur, some of which exhibit the charac- 

 ters of sanidine. 1 Numerous well-developed oligoclase-crystals are 

 sometimes enclosed, arranged in parallel growth near the margin, 

 together with such early products as apatite, sphene, zircon, and 

 biotite. A marginal intergrowth with quartz occasionally forms 

 a micropegmatitic border. 



Among the felspars of the second generation of phenocrysts, 

 orthoclase again appears in varying abundance, seldom exceeding 

 .3 mm. in length, but showing more perfect outlines than the earlier 

 set. In some rocks near the granite it is partly posterior to the 



1 Q. J.G.S. vol. xlvii (1891) p. 288. 



