﻿Vol. 66.] DYKE AT CROOKDENE (NORTHUMBERLAND). 5 



same material as the bars, and the whole is a skeleton crystal of 

 augite. There is a third mode of occurrence in which the bars on 

 each side of an axis (either of felspar or of augite) curve up on one 

 side and down on the other, producing a set of S-shaped curves of 

 augite united by a stem (see PL I, fig. 2). Similar forms have 

 been described by Mr. E. B. Bailey, 1 though no mention is made in 

 his paper of a felspar axis to the skeleton augites. 



Fig. 1. — Colly well Dyke rock, showing acanthus-like strands of 

 imperfectly individualized augitic material, lying betiveen, or 

 curving round, the felspars. 



[Magnified 650 diameters. Grains of iron oxide are accumulated along the 

 edges of augite strands, or in the glassy spaces around them.] 



Besides these definite skeleton forms, there is a yet more ele- 

 mentary stage of growth. The augite seems to have segregated 

 and adopted the curved forms characteristic of the dyke, but 

 definite separation has not yet taken place, and thus the presence 

 of augite is indicated only by the beautiful acanthus-like curves 

 in the glass (fig. 1). This form of elementary crystallization is best 



1 ' On Two Spherulitic (Variolitic) Basalt Dykes in Ardinuchnish, Argyll' 

 Trans. Geol. Soc. Edin. vol. viii (1904) p. 363. 



