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greenish grey, granular rock, having a specific gravity of about 2.95. Not 

 unfrequently it shows lustre-mottling. Under the microscope it is seen to 

 have consisted originally of large irregular ophitic masses of augite, 

 columnar (lath-shaped in section) plagioclase, and irregular plates of 

 ilmenite. The lustre-mottling is due to the fact that the ophitic masses 

 of augite are penetrated by felspar. The ilmenite is almost always more 

 or less changed to leucoxene, which is in some cases transparent and 

 possesses the refraction and double refraction of sphene. The felspars are 

 cloudy and frequently contain spots and patches which appear snow-white 

 by reflected light and opaque or nearly so by transmitted light. The 

 general absence of olivine from these rocks is an important and character- 

 istic feature. The ophitic dolerites of Carboniferous and Tertiary age 

 generally contain this mineral. That these rocks are thoroughly basic in 

 composition, notwithstanding the absence of olivine, is shown by their 

 high specific gravity. They are, therefore, not allied to the andesites 

 either in chemical composition or texture. Good illustrations of this type 

 may be seen in Cwm Orthin, near Ffestiniog ; at Y-Gesell, Moel-y-Gest and 

 Garth near Portmadoc ; at the Gimlet rock, (Careg-y-rimbill), near 

 Pwllheli ; at Forth Dinlleyn and other places in the Lleyn Peninsula ; 

 near the summit of Ehobell Fawr, six miles N.N.E. oi Dolgelly ; and at 

 several points in the immediate neighbourhood of Dolgelly. These rocks 

 are perfectly typical ophitic diabases. 



It has already been stated that the intrusive sheets have been affected 

 by the forces which have folded and faulted the sedimentary rocks. As 

 a consequence of this we find that the diabases occasionally become 

 schistose. There is no evidence at present that in any part of North 

 Wales this action has gone on to such an extent as to have converted large 

 masses of diabase into schistose rocks ; but that schists have been 

 developed locally in connection with the faulting of the diabase is certainly 

 proved. The point is one of some importance, because it will be shown later 

 on that a similar action has taken place in tho West of England on a more 

 extended scale. 



A good illustration of the local development of schist from diabase 

 may be seen in the quarry at Garth, near Portmadoc. The massive diabase 

 is traversed by a fault. In the neighbourhood of the fault it has been 

 converted into a fissile chloritic schist. The normal diabase belongs to the 

 type already described. Very little of the augite has escaped alteration. 

 It is now mostly represented by aggregates of chlorite which give under 

 crossed nicols the pale bluish shimmering light so characteristic of this 

 mineral. The chloritic aggregates are often seen to be penetrated by the 

 lath-shaped plagioclase exactly in the same way as the original augite. A 

 good deal of calcite dust is scattered through the rock and quartz grains, 

 probably of secondary origin, are also present. The passage of the massive 

 rock into the schist is accompanied, as one would naturally expect, by the 

 gradual obliteration of the distinctive micro-structure of the original rock. 

 The chlorite-aggregates are still recognizable in the most perfect schist, but 

 they occur as flat ragged lenticles, their broadest sections lying in the plane 



