356 ' MR. p. LAKE AND PROF. S. H. REYNOLDS ON" [Sept. I912, 



Tocks and the dolerite of the tongue. These are probably con- 

 temporaneous veins — residual portions of the magma analogous to 

 the pegmatite veins in granite ; they, however, depart from the 

 usual rule in being finer-grained than the rocks with which they 

 are associated. The veins have a specific gravity of 2'86 and a 

 silica-percentage of 50* 40. The rock which they penetrate has 

 a specific gravity of 2*87 and a silica-percentage of 50-0. The 

 coarsest dolerite is met with in the south central portion of the 

 mass, to the east of the little pond. South and west of the main 

 mass of Mynydd-y-Gader are many small patches of dolerite, 

 lying in the midst of the rhyolitic and ashy beds. The most 

 important of these are marked upon the map (PI. XXXV), but 

 many are too small to be indicated. 



Essential constituents of the dolerite.— In the coarser 

 dolerites augite occurs as a rule in fresh, pale-brown, brightly 

 polarizing, highly ophitic plates. It is frequently not pleochroic, but 

 occasionally (163) shows faint pale-green and brown tints. Neither 

 diallagic modifications nor hourglass structure were met with, and 

 twinning is not common. 



In many of the fine-grained dolerites the augite is replaced by 

 iibrous amphibole, in others by a chloritic mineral. In some cases 

 the augite is replaced by calcite. No original hornblende has been 

 found in any of the Dolgelly dolerites ; nor has any rhombic pyroxene 

 or olivine been met with in any of the Dolgelly rocks. 



Felspars. — These are always of earlier origin than the augite. 

 The prevalent felspar, labradorite, is often more or less completely 

 replaced by epidote or by calcite. The altered felspars are some- 

 times bordered by fresh secondary material. 



Iron-ores always play a very prominent part, and are some- 

 times original, sometimes secondary constituents. Ilmenite is the 

 prevailing mineral, and may occur in large skeleton crystals or in 

 irregular patches. It invariably shows the usual alteration into 

 leucoxene. Occasionally it is intergrown with magnetite, which 

 has been met with at a considerable number of points, but is far 

 less common than ilmenite. 



Accessory minerals and alteration - products. — The 

 characteristic alteration-product of the dolerites is epidote, as was 

 noted by Cole & Jennings.^ But rarely is a rock met with in 

 which epidote is not conspicuous in microscopic sections ; and in 

 some rocks (216) it is perhaps the most prominent constituent. It 

 occurs, as a rule, in brilliantly-polarizing, often well-cleaved prisms, 

 which are sometimes (163) arranged in a radiating fashion. 



Apatite is decidedly uncommon, having been met with in only a 

 few sections (297). 



Although leucoxene is almost universally present, sphene has 

 been noticed only at one spot (214). 



Pyrite is met with at various points (215, 235, 180), but is not 

 a very common accessory mineral. 



' Q. J. G. S. vol. xlv (1889) p. 432. 



