﻿592 ME. J. V. ElSDEN ON THE IGNEOUS ROCKS [A.Ug. I905, 



presence of enstatrt© is Jearly established, and the rocks can be 

 assigned to the class oi anstatite-diabases. 



At Penclegyr, to the east of Porth Gain, is a large boss-like 

 intrusion which has been extensively quarried. The rock is bluish- 

 grey and of a more compact character than that of Penberry Hill. 

 Slices from different parts of this mass show two principal 

 modifications. In the first variety, which is the rock exposed in 

 the quarry on the northern face of the cliff, there is a close resem- 

 blance to the rock of Penberry Hill. The minerals present are 

 apatite, ilmenite, augite, felspar, rhombic pyroxene, and quartz. 

 Apatite is abundant. The augite clearly precedes both the felspar 

 ^and the rhombic pyroxene. It is granular, and is included in, or 

 moulds, both the last-named minerals. Rhombic pyroxene, however, 

 crystallized later than the felspar, by which it is penetrated. 

 It occurs in rather ragged rectangular sections, and is faintly 

 pleochroic. A large portion of the felspar has rather a low refractive 

 index and a symmetrical extinction of nearly 20° in twins on the 

 albite-plan : it is, therefore, probably albite. A little orthoclase of 

 later crystallization appears to be present. Quartz is fairly abun- 

 dant in small interstitial grains, clusters of which extinguish simul- 

 taneously, but there is no micropegmatite. From its greater acidity 

 this rock may therefore be classed as a quartz-enstatite diorite. 



In another variety of the rock augite is completely wanting, 

 .-and quartz seems to be present in reduced amount. The bulk 

 of the rock consists of plagioclase and a pale yellow pseudomorph 

 after enstatite. The felspar resembles the species above mentioned, 

 and probably is mainly albite, but a little orthoclase is doubtfully 

 recognizable. It preceded the enstatite, which is interstitial to it. 

 Apatite is moderately abundant, and there is some opaque iron-ore 

 in rather irregular patches. I made a partial chemical analysis 

 -of this rock, with the following results : — 



Per cent. 



Silica 61-45 



Alumina, iron, etc 24'80 



Lime 085 



Magnesia 2'91 



(The specific gravity of the rock is 2 - 68.) 



The summation leaves about 10 per cent, for alkalies and water. 

 "The small proportion of lime is significant, and explains the absence 

 of augite in this variety, as well as the character of the bulk of the 

 felspar. Taken in conjunction with the fact, that in the main mass 

 the augite crystallized before the felspar, the explanation might be 

 that this portion of the magma has been derived by a process of 

 liquation from another portion in which augite had already crys- 

 tallized ; or, perhaps, it may represent a magma from which augite- 

 crystals had been removed by gravitation to a lower stratum. I am 

 at a loss for a class-name for this albite-enstatite rock, which 

 •seems to be of rather an abnormal type. (See Pi. XXXIX, fig. 4.) 



