354 Hiss I. H. Lowe — Igneous Rocks of Ashprington. 



usually have a parallel arrangement due to flow. All the felspars 

 are much decomposed, a few still show signs of repeated twinning, 

 the angles of symmetrical extinction of these indicate oligoclase or 

 albite-oligoclase. The refractive index, when compared with that 

 of the Canada balsam, is, as far as can be determined, in agreement 

 with this identification. Skeleton crystals of ilmenite occur in the 

 rock from Stancombe Linhay, and are also irregularly developed in 

 other diabases. Apatite is rare, and is only present in the diabase 

 from Down's Hill quarry ; even in this rock it is slight in amount. 



All the rocks are very much decomposed ; the resulting products 

 include chlorite, epidote, granular sphene, actinolite, white mica, and 

 a mosaic of a clear colourless mineral, probably quartz. The chlorite 

 occurs very abundantly in decomposed felspars, both in veins 

 traversing the crystals and in the felspar itself ; it also forms 

 elongated irregular patches in all the rocks. Epidote is abundant 

 in flocculent aggregates of fine granules both in the felspars and 

 throughout the slides, and also as distinct crystals. Actinolite is 

 present in some specimens in the form of scattered needles, in others 

 forming a fringe from the edge of augite crystals. In one specimen 

 the development can be seen to have taken place to such an extent 

 that practically the whole section consists of patches of actinolite, 

 the needles in each patch being parallel and extinguishing 

 simultaneously. Brown hornblende is very rare, but a few small, 

 ill-defined flakes occur in a slice of the rock collected from the old 

 quarry south of Harbertonford. Granular sphene is associated with 

 leucoxene as the result of the decomposition of ilmenite. Secondary 

 iron oxide in the form of minute irregular patches can be observed 

 in all the sections ; in much decomposed specimens earthy limonite 

 frequently occurs as the material filling the vesicles which have 

 been exposed to surface weathering. White mica in small flakes 

 and a fine mosaic of a colourless mineral occur in the decomposed 

 felspar. Kaolin as a decomposition product of felspar is especially 

 well developed in the rock of Down's Hill quarry. The section in 

 this exposure showed at the east end a compact, grey, amygdaloidal 

 rock with phenocrysts of felspar, becoming more vesicular towards 

 the west. The rock is cut by almost vertical faults, along one of 

 which a development of kaolin occurred, forming a highly cleaved 

 white flaky mass a few inches in thickness and stained in parts with 

 iron. It seemed to be a final stage of decomposition of the rock, 

 which is very rich in felspar. 



Calcite is conspicuously absent in these rocks, except in the cases 

 where they overlie limestone bands, as at the old quarry near 

 Tuckenhay. Its absence as a decomposition product of felspar gives 

 further support to the view that the felspar is not of a very basic 

 species. 



Summary of the Decomposition of the Diabases. 

 All the diabases of the Ashprington area are intensely altered ; 

 the modifications which the rocks have undergone have resulted in 

 three types of change. 



(1) Mineralogical alteration by which the original constituents 

 of the rock have been replaced by secondary minerals. 



