128 HE. ALFRED HABEER ON THE CARROCK EELL [May 1 895, 



of felspar and quartz, not essentially different from the centric- 

 type of micropegmatite. Indeed, the evidence put forward by Whit- 

 man Cross 1 and Iddings 2 amounts almost to demonstration. The 

 Carrock Fell rock exhibits, better than any other with which I am 

 acquainted, the various stages of gradation from a coarse and 

 irregular micropegmatite to spherulitic intergrowths of a delicacy 

 surpassing the resolving power of the microscope. These structures 

 collectively will be conveniently designated ' graphic ' structures. 

 The name ' granophyre ' applied to rocks of this type is perhaps too 

 well established by usage to be abandoned. 



When the intergrowth is on a relatively coarse scale, it is usually 

 rude and irregular. Indeed, some specimens of the rock show little 

 or no graphic structure at all, the felspar and quartz forming an 

 irregular mosaic. In this case the quartz tends to occur partly in 

 larger crystal-grains, and the rock approximates to some quartz- 

 porphyries. This, however, is exceptional, almost all the slides exa- 

 mined showing at least a partial micrographic structure. The more 

 finely-textured micrographic intergrowth always shows a regular 

 arrangement. Sometimes the arrangement is linear or parallel ; 

 more frequently it is centric ; but in all cases a large part of it 

 stands in definite relation to the porphyritic crystals of oligoclase, 

 forming a more or less regular frame round each crystal. In very 

 many cases it can be verified that part of the felspar of the inter- 

 growth is in crystalline continuity with the felspar-crystal which 

 has served as the nucleus for its growth. The appearance is as if 

 the original crystal had continued to grow throughout the final 

 stage of consolidation of the magma, enclosing the residual excess 

 of silica as intergrown quartz. In some instances a line of Carlsbad 

 twinning can be traced from an oligoclase-crystal through the 

 enveloping framework. It is clear that much of the felspar in the 

 micrographic intergrowth must be of a plagioclase variety. In some 

 Caernarvonshire granophyres I have found that the felspar of the 

 micrographic intergrowth is in continuity, not with the plagioclase- 

 crystal which forms the nucleus, but with an extremely narrow 

 border of orthoclase investing the plagioclase. 3 In the Carrock Fell 

 rock I have not observed this peculiarity, though it is seen in some 

 specimens from Buttermere. 



The more delicate the micrographic intergrowth, the more marked 

 is the tendency to a ' centric ' or radiate arrangement, with or with- 

 out a nucleus of an earlier crystal. With increasing fineness also 

 the sectors within which the felspar of the intergrowth extinguishes 

 simultaneously become narrower, until they are represented between 

 crossed nicols merely by dark rays when their direction makes a 

 small angle with one of the cross-wires. The felspar of the inter- 

 growth takes on more markedly the form of radiating fibres, and 

 seems to make up a larger proportion of the whole. This latter 

 appearance is perhaps illusory, but it is certain that the aspect of 



1 Bull. Phil. Soc. Washington, vol. xi. (1891) pp. 411-444. 



2 Ibid. pp. 445-464. 



8 ' The Bala Volcanic Series of Caernarronshire,' 1889, p. 48. 



