66 MUs M. K. Heslop 8f R. C. Burton— 



a "veritable mikrolithen filz, a closely-matted aggregate of minute 

 felspar microliths ", ami this is, in general, the character at the borders 

 of the Bohim and Cockfield laccolites ; it is in strong contrast to the 

 almost homogeneous glass with its small supplj- of felspars, which is 

 discussed in this paper. It is true that a little earlier in the same 

 paragraph as the above quotation we find that "in one instance only 

 was an isotropic glass observed, and then the glass was seen to contain 

 longulites and globulites similar to those in the Eskdale Dyke". 

 This cannot be said of the Teesdale tachylite, for although it contains 

 crystals they have, even the smallest (which are seen penetrating 

 bubbles) been formed before they readied their present position, 

 where there is no evidence of crystallization having proceeded ; for 

 the glass, where undevitrified, is absolutely homogeneous, so far as 

 can be determined by present microscopic appliances. 



It will be convenient here to discuss a few points concerning the 

 petrology, and the relations between the tachylite and the glass of 

 the groundmass of the dyke. Some very interesting conclusions may 

 be drawn from the observations on the connexion between the cooling 

 cracks and the crystals of the tachylite. It has been pointed out that 

 two generations of crystals occur — 



1 . The porphyritic felspars and augites. 



2. The groundmass felspars and augites, few and small, and it has 

 been shown that all these crystals were formed prior to the tachylite 

 itself, which was undoubtedly produced in situ by rapid cooling at 

 the edge of the dyke. The history of the magma is, therefore, as 

 follows. 



The porphyritic crystals were formed first, and were floated up by 

 the magma from a fairly low depth, when it was intruded into the 

 fissure ; but on the way cooling began, possibly during periods of 

 quiescence, and new crystals of a later generation were formed, and 

 probably the zoning of the felspars commenced. The residual magma 

 was rapidly cooled where it came into contact with the walls of the 

 fissure, and the tachylite was formed ; this glass, then, we may term 

 the ' primary groundmass ', enveloping crystals of two generations, 

 the later generation being one of extended duration. But in the 

 centre of the dyke crystallization proceeded, yielding the groundmass 

 crystals and continuing the zoning of tlie felspars ; finally, we had left 

 the 'later groundmass', which accoi'ding to Stock's analysis is of 

 acidic composition. One glass is thus, in a way, formed from the 

 other, and the groundmass of the dyke is a product of difi^erentiation 

 by crystallization from a material havinjj the same composition as the 

 tachylite, which is an intermediate rock. Moreover, the glass of 

 the groundmass analysed by Stock is much richer in alkalies than 

 the tachylite or the ordinary variety of the dyke, so that it is not 

 surprising to find that the outer zones of the porphyritic felspars are 

 of a more alkaline type than the inner zones ; and even whole crystals 

 of albite occur. Thus, as differentiation by crystallization proceeded 

 we had felspar material of a more alkaline type crystallizing out, and 

 it is evident that the albite crystals wrre the last formed; and from 

 their ill-defined outlines it is inferred that they had not time to fully 

 develop. 



