242 HARKER : NOTES ON NORTH OF ENGLAND ROCKS. 



decomposition, but with this occur portions which are clear and 

 colourless, and give bright pink and green polarisation-tints like those 

 of white mica. 



A slice from Muncaster Quarry, Ravenglass [952] shows most of 

 the characters described above, including fine examples of the micro- 

 perthite intergrowth. Specimens may be collected from other parts, 

 however, which show a departure from the granitic type along one or 

 other of the two lines of gradations noted above. One slide, for 

 instance, shows a very typical micropegmatite [954] : another from 

 Brantrake Moss is a microgranite very similar to that of St. John's 

 Vale [748]. 



An analysis of the Eskdale granite agrees roughly with the 

 composition : 27 per cent, of quartz, 20 of orthoclase and microcline, 

 45 of albite and albite-oligoclase, and 8 of decomposition-products 

 of felspar, biotite, etc. 



(xii) Diabase or Doleriie of Castle Head., Keswick. — This intrusive 

 mass is of interest, according to Mr. Ward, as probably marking the 

 site of one of the Ordovician volcanoes which gave vent to the lavas 

 of the Borrowdale series. The rock shows an intimate admixture 

 of black augite and light-coloured felspar stained with greenish 

 secondary-products. In this some of the little augite crystals stand 

 out prominently enough to impart a porphyritic appearance. 



Under the microscope [755] these crystals give outlines 

 rectangular in longitudinal and octagonal in transverse sections, the 

 characteristic cleavage-cracks parallel to the faces of the prism being 

 well marked. The polarisation-tints are of a high order. The crystals 

 are frequently twinned, a pecuHarity brought out clearly between 

 crossed Nicols, where the two individuals of a twin give different 

 tints and extinguish the light in different positions. In addition to 

 this, two or three crystals are usually grouped together so as to 

 interfere with one another's growth. 



The ground-mass has been composed of plagioclase felspar 

 and augite, but the latter mineral is totally destroyed, and the 

 little oblong crystals of felspar are for the most part deeply altered. 

 Little irregular patches and skeleton-crystals of black opaque 

 magnetite are scattered about the slice. The secondary products 

 include clear quartz granules, dust of calcite, stains of red-brown 

 iron oxide, a pale chloritoid material with low polarisation-tints, and 

 serpentine. The last-named substance is chiefly collected in little 

 veins in the rock, and has a fibrous structure, the fibres being set 

 perpendicular to the walls of the vein. The serpentine gives low 

 colours of polarisation, being nearly dark between crossed Nicols 

 and extinguishing parallel to its fibres. 



Naturalist, 



