Vol. 62.] BUTTEEHEEE AND ESTNEEDALE GEANOPHYEE. 271 



kiud is present, the tendency to a centric arrangement of the quartz 

 and felspar is not so marked as in many granophyres. 



Micropegmatifce also occurs very commonly in an interstitial 

 manner, between more or less idiomorphic crystals of felspar 

 or ferromagnesian minerals. Several different kinds of micro- 

 pegmatitic structure may be distinguished, besides the fringing 

 variety above mentioned : for instance the platy, the triangular, and 

 the feathery types. Some of these apparent differences are doubt- 

 less due to the varying directions in which the sections are cut, 

 and this almost certainly applies to the triangular and platy types. 

 Some examples have a very strong resemblance to certain forms 

 of perthitic structure ; and, in some specimens, examination with 

 a higher power shows that, what appear to be homogeneous and 

 idiomorphic felspar-crystals under a low power, are in reality ex- 

 tremely fine- textured intergrowths of quartz and felspar. 



It has been pointed out by many writers that mixtures of two 

 substances, in a proportion approximating to the eutectic ratio, 

 frequently show structures resembling micropegmatite ; and it is 

 now believed that ordinary micropegmatite is a eutectic of quartz 

 and felspar. According to Prof. Vogt, 1 the eutectic ratio for 

 quartz and felspar is 26 : 74 ; his figures apparently apply to ortho- 

 clase, and it is possible that the ratios may be slightly different 

 for plagioclase. 



The eutectic mixture would be the last to crystallize ; and so we 

 should naturally expect to find micropegmatite in the inner parts 

 of the mass, which remain longest in the liquid state. 



It appears, therefore, that in the case of the Ennerdale Grano- 

 phyre the conditions of cooling were such that crystallization went 

 on from without inwards, leading to the successive supersaturation 

 of the magma for the different constituents, according to the 

 principles laid down by Prof. Morosewicz and others. The course of 

 crystallization was such that the still molten residue approximated 

 more and more closely to the composition of the eutectic, and 

 therefore finally solidified as micropegmatite. 



This is in accordance with the idea put forward by Prof. Moro- 

 sewicz 2 and Dr. Teall. 3 These processes, however, seem to be 

 controlled by other factors, such as rate of cooling and pressure. 

 The true deep-seated plutonic rocks, which have consolidated under 

 a great thickness of rock-cover, do not show much graphic structure. 

 On the other hand, this structure is very well-marked in many 

 rocks which appear, from independent evidence, to have consolidated 

 under a comparatively-thin covering, although occurring in large 

 masses : as, for example, many of the Tertiary intrusions of North- 

 Western Europe. This suggests that pressure is probably an 

 important factor in the case, and that pressure alone, or pressure 

 and rate of cooling conjointly, determine in any particular instance 

 whether an acid magma shall crystallize as a granite or a granophyre. 



1 'Die Silikatschmelzlosungen' pts. i & ii, Vidensk. Selsk. Skrifter, 1903, 

 no. 8 & ibid. 1904, no. 1 ; see also review by A. H. in Geol. Mag. 1905, p. 132. 

 - Tscbermak's Miu. Petr. Mitth. n. s. vol. xviii (1899) p. 1. 

 3 Presidential Address, Quart. Journ. Geol. Soe. vol. lvii (1901) p. lxxv. 



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