432 MR. J. PARKINSON ON DEVITRIFICATION [NOV. 1903, 



structure which characterizes the lower compact obsidian fades 

 away in the upper scoriaceous layer. 



Since an eutectic favours crystallization, the inference is clear 

 that, either obsidian-glass is not an eutectic, or is an eutectic of 

 soda-felspar (albite or oligoclase), potash-felspar, quartz, and water 

 suddenly cooled. 



Specimens taken from the lowest part of the cliff and from the 

 centre of the columns, where it is safe to assume that cooling pro- 

 ceeded most slowly, show no sign of primary devitrification in a 

 thin section. Moreover, it must be exceedingly rare for a magma 

 devoid of porphyritic crystals (which might utilize constituents in 

 excess) to possess eutectic proportions. Hence we may conclude 

 that the latter condition did not obtain. 



Types of Primary Devitrification. 



The types of primary devitrification and the causes which govern 

 their formation are discussed by Prof, Bonney in Part II of this 

 paper, so that one or two special examples are all that need be 

 mentioned at this stage. 



In two slides, I believe that I detect an arrangement of minerals 

 which suggests that an eutectic zone may follow the crystallization 

 of an overplus of quartz. 1 In one case the main part of the rock 

 consists of secondarily-devitrified glass ; in tbe other ( ' porphyry- 

 pitchstone ' from Spechthausen) it is composed of small spherulites, 

 and here the so-called ' eutectic zone ' has a spherulitic appearance, 

 differing from the surrounding material merely in being slightly 

 coarser. 



In this slide the ' eutectic ' structure usually forms isolated 

 patches, or a band, of more or less micrographic material, in which 

 either quartz or felspar may predominate, surrounded by a more 

 homogeneous matrix. In a third instance, from Anne Port (Jersey), 

 the structure closely resembles an illustration given by Mr. J. E. 

 Stead 2 which shows three contiguous grains of a metal-ingot con- 

 taining 1-8 per cent, of phosphorus. A triangular patch of eutectic 

 occurs in the space between the three grains. In the Anne-Port 

 slide the resemblance is heightened by the body of the rock, which 

 surrounds the semi-micrographic patch, breaking up between crossed 

 nicols into the mosaic of grains (referred to on a later page as 

 ' patchy devitrification ') which simulate the components forming the 

 grains of the ingot. 



Another instance from the same island (south of Vicart Cliffs) 

 presents similar structures (PI. XXVI*. fig. 2). In this the early 

 crystallization of the superfluous silica is well shown, bordered by 

 an intergrowth of quartz and felspar, crystals of the latter pro- 

 jecting into the central grain or group of grains. In the body of the 

 rock the differentiation of the constituents is barely perceptible, 



1 That is, a zone with a micrographic arrangement of parts, in this instance 

 not strictly marked off either from the quartz on the one hand, or from the 

 outer rock on the other. 



2 Journ. Iron & Steel Inst. vol. lviii (1900) pi. iii, no. 2. 



