﻿Vol. 66.~] DYKE AT crookdene (noethtjmberland). 13 



ment in groups of four or five. Augite, either in large crystals 

 or in granular aggregates, is gathered round the felspar. Both 

 these minerals are embedded in a glassy base which is rendered 

 almost opaque by the presence of granular iron oxide and skeleton 

 augite in great abundance. It would seem that the bulk of the 

 iron oxide is in a globulitic condition : for, although small square 

 sections and skeleton crystals of it are common, they are not 

 numerous. 



The rock is characterized by the presence of many large porphy- 

 ritic felspars and amygdaloids of calcite. The latter vary consider- 

 ably in size, and show semi-lunar residues consisting of dark glass 

 in which some devitrification has usually taken place ; while some 

 are entirely filled by ground-paste with skeleton crystals. There are 

 green patches, probably pseudomorphs after olivine, but these are 

 rare ; and pockets of green glass, such as characterize the Morpeth 

 Dyke, are practically unknown. 



The porphyritic felspars are large, and are mostly intergrown in 

 groups of two or three, single individuals being rare. There is 

 invariably a zone of later felspar material which surrounds the group 

 as a whole, but is not carried between the individuals which comprise 

 it. This point was noticed and emphasized by Dr. Teall, and he also 

 observed (Quart. Journ. Geol. Soc. vol. xl, 1884, p. 233) that 



' the internal boundaries of the crystalline particles do not show definite 

 faces, although the external boundaries usually do.' 



That is, the proper faces of the crystals are furnished by the later 

 additions of felspar material. These porphyritic felspars are usually 

 traversed by roughly parallel cracks, which do not appear to bear 

 any definite relation to the crystal outlines — they are not the usual 

 cleavages. In some cases the cracks have been filled by calcite, 

 but in the freshest crystals they consist of felspar material, and are 

 only distinguishable between crossed nicols as bands of brilliant 

 colour. 



These bands correspond to the large cracks which occur in 

 the felspars of the aggregates in the other dykes, and possibly 

 represent directions of weakness which might, in other circum- 

 stances, have become cracks. The interfere nee- colours are chiefly 

 of the rainbow type, such as are produced by thin films of strained 

 material. 



These porphyritic crystals were isolated and analysed by Mr. G. E. 

 Stead ( p. 7), and proved to be ' closely allied to anorthite.' In 

 all respects — in composition, in twinning, in arrangement and 

 character of inclusions, and in optical properties, they are identical 

 with the felspars of the aggregates in the other dykes; and Dr. TealPs 

 summary of their most salient characteristics may be applied to 

 the felspar aggregates of the Crookdene and Collywell Dykes : 



' The internal relations of the individuals forming a group are those of 

 plutonic rocks (e. g. gabbro), whereas the external relations of the same 

 individuals are those of volcanic rocks,' (Geol. Mag. dec. iii, vol. vi, 1889, 

 p. 481.) 



