70 DE. A. E. DWEEEYHOT7SE OX INTETJSIVE BOCKS [Feb. 1909, 



However this may be, the fact that these intimate intergrowths 

 occur points to a simultaneous separation of their constituents, 

 a phenomenon which demands an explanation. 



The presence in a rock of more than one pair of minerals which 

 owe their origin and association to the solidification of an eutectic 

 mixture, will probably be found to be due to the successive exist- 

 ence of a number of such mixtures in the magma at different stages 

 of its cooling. It may well be that, as the magma cools, an eutectic 

 association takes place, that this gives rise to the simultaneous 

 solidification of a pair of minerals, and that at lower temperatures 

 certain phases may detach themselves and form other eutectic asso- 

 ciations, stable at these lower temperatures, thus breaking up the 

 earlier one, and in their turn giving rise to other intergrowths. 



It is now well known that changes in the molecular constitution 

 of steel and alloys take place during cooling after solidification, one 

 of these being the breaking-up of a solidified eutectic into a coarser- 

 grained mosaic of its constituent compounds, with the obliteration 

 of the characteristic eutectic structures. This change can be de- 

 monstrated both by cooling-curves and by microscopic examination, 

 and the non-occurrence of micropegmatitic structure in many of the 

 q\iartz-felspar ground-masses may be due to some such change. The 

 experimental investigation of these matters is not easy. The high 

 melting-points of the substances are in themselves a difficulty, and 

 the extremely slow rate of cooling necessary to ensure the crystal- 

 lization of the silicates renders the study of cooling-curves and 

 volume-changes extremely difficult, if not well-nigh impossible. 



To return to the question of the acid margins of the granite. It 

 is, of course, well known that most batholiths, especially those of 

 basic composition, vary considerably in composition as between 

 their central and peripheral parts, the edges of the intrusion being 

 usually more basic than the interior. 



In the case of the Eskdale Granite, it is my opinion that the 

 reversal of the usual distribution, namely, the occurrence of the 

 more acid rock on the periphery and the more basic in the interior, 

 is due to the magma as a whole having been more acid than some 

 eutectic, probably that of quartz and orthoclase, and the consequent 

 primary separation of quartz until the eutectic proportions were 

 reached, at all events locally. 



The case for the early separation of quartz from the magma does 

 not rest on the marginal phenomena alone : for, as has already been 

 stated (p. 63), the rock contains examples of quartz-crystals enclosed 

 in phenocrysts of oligoclase and other felspars ; while the presence 

 of phenocrysts, similar to those that occur in the quartz -porphyries, 

 in some of the finer-grained varieties, points in the same direction. 

 One cannot, however, expect a complete solution of this problem 

 without further experimental work. 



