﻿630 ME. W. G. FEAENSIDES ON" THE GEOLOGY OE [Aug. I905, 



Ferromagnesian minerals are, so to say, rather at a discount, and 

 are as a rule represented by ramifying veins and good pseudoniorphs 

 of spherulitic pale-green chlorite, which is probably delessite. Some 

 of the pseudomorphs are exceedingly-well formed, and, from their 

 constant octagonal cross-section and elongate outline, are almost 

 certainly after a rhombic pyroxene, which was probably hypersthene. 

 The groundmass of the rocks is always recrystallized and 

 probably always silicified ; when well preserved it shows a parallel 

 or variolitic arrangement of rather stumpy microlites of plagioclase 

 set in a coarsely micropoecilitic or granular mosaic of quartz or 

 untwinned felspar (4653-54 & 4657, 4659). Half-digested frag- 

 ments of some ophitic andesine-bearing rock, quite like the later 

 dolerites of the district, are frequently enclosed in this rock as in 

 the corresponding Stapely rocks of Shropshire (4661). The infilling 

 of the larger vesicles is usually calcite, that of the smaller ones is 

 sometimes calcite, but more frequently quartz or spherulitic chal- 

 cedony. Some of the cavities have both quartz and calcite, together 

 with a little chlorite ; and, in these, it is worthy of note that the 

 calcite is usually external to the other minerals (.4655-56 & 4658, 

 4660). 



The Andesitic Dolerites [24] . 



Just as the hypersthene-andesites tend to follow the horizon of 

 the Great Agglomerate, the andesitic dolerites spread themselves 

 abundantly about the horizon of the unconformity and the Basal 

 Grit. Unlike the andesites, however, they range in a series of minor 

 masses far down into the Cambrian, and in two localities extend 

 upwards as far as the Great Agglomerate, in one place entering 

 the Daerfawr Shales. Being of a rather basic character, they are 

 usually much chloritized and very far from fresh, yet petrologically 

 are quite interesting. They are always non-porphyritic, and generally 

 to some extent ophitic. They contain from 53 to 56 per cent, of 

 silica. Though occasionally slightly vesicular along their edges, 

 they are distinctly hypabyssal in the mass of the rock, and range 

 almost to gabbros in the centre of the larger laccolites. Yesicles, 

 when present, are always filled with calcite (4664). Although I 

 suspect, I have not been able to prove, chemical differentiation 

 between the centres and edges of masses : such variations in silica- 

 percentages as do occur seem irregular in their distribution. 



Under the microscope, it is seen that the rocks belong to a 

 rather acid type of dolerite, and that the ophitic structure is by 

 no means constant in them. 



The great bulk of the rock consists of a plagioclase-felspar with an 

 extinction-angle ranging on symmetrically-cut albite-twins up to 

 20°, but rarely higher. On powdering the rock, it was found that 

 nearly all the felspar floats off in a mercury-and-potassium iodide 

 solution of specific gravity 2*68, sinking again when the specific 

 gravity is reduced to 2*64. Cleavage-fragments so obtained showed 

 extinction-angles never greater than 8°, and mostly gave only 1° 

 or 2° upon the second cleavage. The felspar is, therefore, almost 



