﻿582 ME. J. T. ELSDEN ON THE IGNEOUS KOCKS [Aug. 1905, 



The large mass lying along the northern coast of Strumble Head 

 differs from this type in being a uniformly fine-grained 

 basaltic rock, distinctly columnar at Pen Anglas, and in places 

 tachylytic. Along the southern margin, from Penrhiw, near Good- 

 wick, to Porth Melyn, it is amygdaloidal or vesicular, and often it 

 closely simulates a true surface-flow. At its western end it has been 

 considerably sheared, as maybe seen at Trevishec and at Rhos-yn-wen. 

 On the Geological-Survey map this rock is merged in the Llanwnda 

 rock, but I saw no actual junction in the field, nor any sign of the 

 passage of this fine-grained basalt into the very coarse Llanwnda 

 type : Mr. Cowper Heed, in fact, refers it to a later period. 



We may now proceed to trace a variation in the basic type, referred 

 to above, which appears to possess considerable interest in view of 

 the character of the intrusive rocks occurring farther south. For 

 this purpose, it will be convenient to describe in more detail the 

 group of intrusions lying immediately to the west of Llanwnda, 

 between Pont Iago and Porth Melyn. This group forms a well- 

 defined range of hills rising into the conspicuous summits of YGarn, 

 Garn Eolch, Garn Gilfach, Garn Pechan, and Garn Fawr. 



If we begin the examination of this ridge at Y Garn near Pont 

 Iago, we find that the rock, although microscopically indistinguish- 

 able from the medium-grained Llanwnda type, shows under the 

 microscope the gradual appearance of a rhombic pyroxene. The 

 opbitic augites also give place to a more pronounced idiomorphic 

 type. Some of the crystals of augite appear to be of earlier con- 

 solidation than the felspar; and a later generation, penetrated by 

 felspar-laths, includes remnants of a rhombic pyroxene, some of 

 which is intergrown with the earlier generation of augite. The 

 rhombic pyroxene has undergone the usual bastite- alteration, and 

 includes numerous granules of brown sphene. 



At Garn Bolch a rock of coarser grain shows, in addition to the 

 above-described characters, zoned felspars, and also felspars of 

 a later generation with a lower refractive index. These rocks, 

 which approximate to enstatite-diabase in character, 1 are 

 penetrated by dykes of a finer-grained rock, with granular augite, no 

 enstatite, and a considerable proportion of magnetite, the last-named 

 mineral being replaced in the main mass by ilmenite. 



Proceeding to Garn Gilfach we find a typical dolerite, with 

 ilmenite, magnetite, and pbenocrysts of felspar and augite, the 

 groundmass containing square sections of untwinned felspar, augite, 

 and an abundance of magnetite. This rock bears a close resem- 

 blance to the dykes which penetrate Garn Bolch. 



We now come to Garn Fechan, where the dominant rock is the 

 variolite, so fully described by Mr. Cowper Reed. It is important 



1 The term enstati te is used in a general sense, to denote both the unaltered 

 and the altered forms of this mineral, and to include also the more ferriferous 

 varieties, known generally as bronzite and hypersthene. 



