G. W. Tyrrell— Tntrtmons of Kllsijth-Croy Didrict. 365 



Origin. — As I have not yet completely investigated the literature, 

 I do not know whether these generalizations are true of every occni'- 

 rence of diabase with micropegmatite. There are obvious difficulties 

 in the way of proving their truth. If they are true, then it is certain 

 that these facts have not only a very important bearing on the petro- 

 genesis of diabase with micropegmatite, but also on petrogenesis in 

 general. In Scotland both plateau basalts and the great dykes of 

 diabase with micropegmatite appear to be connected with the great 

 block fractures due to radial contraction of the earth. If this relation 

 can be shown to hold elsewhere, it may open up attractive speculation 

 as to the association of certain types of igneous rock, and perhaps 

 certain modes of differentiation, with particular kinds of earth-move- 

 ments.^ In this field the recent work of Becke and Prior is sufficiently 

 well known. 



In the lack of synthetic data bearing on the origin of granophyric 

 diabase, any theory must be regarded as purely speculative. In his 

 very suggestive papers on the Mechanics of Igneous Intrusion,- 

 Dr. E. Daly has advocated the theory of abyssal assimilation with 

 subsequent gravity differentiation as a mode of origin of many igneous 

 rocks. He believes that these owe their origin to the interaction of 

 a postulated universal layer of potentially-liquid rock of gabbroid 

 composition in the crust beneath the level of no strain, with the over- 

 lying acidic crust, the interaction taking place in chambers formed 

 by ' magmatic stoping'. In one of his papers Dr. Daly hints that the 

 peculiar field relations of gabbro and granophyre, so perfectly displayed 

 in many parts of the United Kingdom, may be susceptible of ex- 

 planation on the abyssal assimilation theory. It is impossible here, 

 however, even to indicate in the barest outline the detailed facts and 

 calculations upon which Dr. Daly bases his theory. 



The suggestion is here put forward that the gabbro-granophja'e 

 melange rocks owe their origin to the interaction of a normal basalt- 

 magma with a highly siliceous country rock, in the manner advocated 

 by Dr. Daly, and that the normal granophyric diabases, with their 

 remarkably constant chemical composition, represent the saturation- 

 point of such a magma with silica. The excess of siliceous matter 

 is believed to be thrown out as a separate body of material usually 

 consolidating as granophyi'e, in a manner analogous to the separation 

 of the excess of a salt in a saturated solution. The theory thus crudely 

 set forth can only be barely indicated, much less discussed, in this 

 place. It depends largely on further synthetic research as to whether 

 there is such a saturation-point for a basic magma in which silica has 

 been dissolved. That granophyric diabase represents a critical point 

 in the history of its magma may perhaps be inferred from the fact 

 that it is almost entirely made up of intergrowths of related minerals. 

 The iron-ore is made up of intergrown magnetite and ilmenite, the 

 pyroxene probably of submicroscopic intergrowths of augite and 



' SeeH. I. Jensen, " Distribution, Origin, and Relationships of Alkaline Eocks " : 

 Proc. Linn. Soc, N.S.W., 1908, vol. xxxiii, p. 491. 



- Anier. Journ. Sci. (4), 1903, vol. xv, p. 269 ; (4), 1906, vol. xxii, p. 195 ; 

 (4), 1908, vol. xxvi, p. 17. Also " Secondary Origin of certain Granites": A.J.S. 

 (4), vol. XX, p. 185. 



