164 T. DAYIE3 ON THE MICROSCOPIC STRUCTURE 



Appendix on the Microscopic Structure of some Dimetian and Pebi- 

 dian Rocks of Pembrokeshire. By Thomas Davies, Esq., F.Gr.S. 



Dimetian. 



1. The rock from near the Church School is seen rnacroscopically 

 to consist largely of quartz in individual crystalline grains, or in 

 groups, thickly distributed through a porphyritic-felsite ground-mass. 

 In fissures and nests in the mass of the rock much chlorite and 

 also some epidote are present. Its affinity to the quartz porphyries 

 is only apparent when examined in thin section under the micro- 

 scope. Here the quartz is seen to be distinctly crystallized, with 

 characteristically rounded edges, and also with inclusions of the 

 ground-mass. The felspar crystals are numerous, and much altered 

 though the indications of banding of a plagioclase felspar are not 

 wanting. 



The ground-mass is microcrystalline throughout, consisting of 

 quite individualized grains of quartz and felspar, and in many 

 parts presents a peculiar structure. Around the quartz and felspar 

 crystals an incipient spherulitic arrangement has taken place, re- 

 sembling to some extent that seen in some rhyolites ; but here the 

 radial texture is modified by a tendency to a dendritic or a branched 

 structure, a peripheral outline being in all cases absent. This 

 structure is also developed from numerous individual centres in the 

 ground-mass, having no apparent nucleus ; it bears no resemblance 

 to the rhyolite of the breccia before described, though it is pro- 

 bably but a modified form of spherulitic structure. Much green 

 chlorite and a black indefinable substance is dispersed through the 

 ground-mass. 



2. This fine-grained, chlorite-spotted rock presents in thin section 

 a completely crystalline ground-mass of felspar and quartz grains, 

 the latter being but in small proportion. The peculiar dendrito- 

 spherulitic structure, shown in the rock from the Church School, 

 is present here also, but is not so distinct. Crystals of quartz are 

 entirely absent ; and but a few crystals of felspar are developed. 

 The green chloritic mineral and the black substance, as in the 

 Church-School rock, are pretty evenly distributed. 



3 a. The dense purplish-black rock in contact with the quartzose 

 rock is seen in thin section to consist of very small crystals of a 

 plagioclase felspar pervading a grey uncrystalline ground-mass, 

 which is perfectly opaque between crossed Mcols. With polarized 

 light very small grains of a strongly polarizing mineral are seen, 

 but they are not sufficiently distinct to admit of determination as 

 to whether they may be olivine or augite. The felspar crystals lie 

 more or less parallel to the line of junction with the quartz rock ; 

 it is to be referred to the basalt group. 



3 b. The altered igneous rock in contact with the quartz rock in 

 this section much resembles that of the preceding, though it is not 

 so dense, and the indications of the more abundant presence of 



