﻿288 ilR. J. Y. ELSDEX OX THE L^^Y I908, 



polj'synthetic lamellation appears to be iudicated by a different 

 extinction-angle in alternate lamellae. 



All the foregoing observations seem to lend support to the 

 perthitic theory. Increased interest attaches to this view in the 

 light of the recent synthesis, by Messrs. E. T. Allen, F. E. Wright, 

 & J. K. Clement, of a monocliuic form of magnesium-metasilicate.^ 

 According to them this substance is tetramorphie, the stable form 

 being the monoclinic magnesia-pyroxene, into which all the other 

 forms pass at temperatures above 1150° C, the change being 

 monotropic. Enstatite was only produced at lower temperatures. 

 Under certain conditions an orthorhombic magnesia-amphibole was 

 formed, with a transition-point at about 400° C, when it passed into 

 monoclinic amphibole. As Ml[. Eouque and Michel-Levy have dis- 

 covered the presence of monoclinic magnesia-pyroxene in meteorites, 

 the question arises whether this substance may not also exist in 

 igneous rocks. Analyses by Dr. Teall,^ and by A. Y. Merian,^ seem 

 to show that the enstatite-molecule is present in these striated 

 pyroxenes; and it is, therefore, not unlikely that the basal striation 

 referred to above may be due to a minute parallel iutergrowth of 

 monoclinic magnesia-pyroxene and augite. TVe may carry this 

 theory still further : Dr. Teall detected in the Whin-Sill rock a 

 pale pyroxene of earlier formation than the augite, and similar 

 observations have been made elsewhere."* I have noted this 

 occurrence in many diabases, and in the St. David's-Head rocks the 

 numerous circular inclusions in many of the augites seem to be of a 

 similar nature. It is possible that all these are the monoclinic 

 form of magnesia-pyroxene. ■ 



Iron-Ores. — The iron- ores are distributed with fair uniformity 

 through all the types, and are never abundant. In no case has any 

 tendency to marginal concentration been noticed. In the most 

 basic variety they are even less conspicuous than in the more acid 

 rocks. They occur mostly in the form of titaniferous magnetite 

 or ilmenite, and are usually more or less altered to leucoxene. 

 In order of separation this mineral is by no means confined to 

 the earliest phase, but is more often moulded upon the felspars, 

 with very irregular outlines, though occasionally exhibiting rhombo- 

 hedral forms. Very commonly it forms long strips bordering other 

 crystals, or patches partially enveloping the felspars. Pyrites in 

 small quantity is present in a few instances. 



Eiotite. — Biotite is confined to the most basic types of rock, 

 and is therefore more commonly found in the Carn-Llidi intrusion. 

 It is always a primary constituent, is never very abundant, and 

 invariably separated later than the felspars, upon which it is 



1 Amer. Journ. Sci. ser. 4, vol. xxii (1906) pp. 335-438. See also J. W. 

 Judd, Quart. Journ. Geol. See. vol. xhi (1886) p. 65 (footnote). 



2 Ibid. vol. xl (1884) p. 649. 



3 Neues Jahrb. Beilage-Band iii (1885) p. 289. 

 ■* W. Wahl, op. cit. passim. 



