208 harker: notes on north of England rocks. 



slide is rotated. Under the same circumstances the longer and 

 narrower felspars exhibit the finely-banded or striated appearance 

 characteristic of plagioclase. They belong to an 'acid' variety 

 (oligoclase), as is proved by their low 'extinction angles'; i.e., if the 

 bands be set parallel to one of the cross-wires, a small rotation 

 suffices to bring one or other set of alternate bands to a position of 

 extinction. 



The clear quartz is dotted with minute inclusions, partly arranged 

 in lines. The abundant flakes of brown mica (biotite) are con- 

 spicuous by their marked cleavage traces and deep brown colour : 

 when rotated over the polariser, without the analyser, they show 

 intense dichroism. The biotite is, however, much affected by 

 decomposition, which often eats along the cleavage -planes, and 

 leaves a green chloritic mineral giving very low polarisation -tints, 

 almost dark, between crossed Nicols. The sphene occurs partly in 

 rounded granular patches, and partly in small acute parallelogramic 

 sections : it is light brown, with moderately bright polarisation-tints, 

 and in ordinary light seems to stand out in relief by virtue of its high 

 refractive index. The black, opaque magnetite is in irregular patches, 

 only occasionally showing the outlines due to octahedral crystals. 



The sphene and magnetite are the first-formed constituents, then 

 the biotite, and lastly the quartz and felspars. The orthoclase is 

 newer than the plagioclase, as is proved by its sometimes enclosing 

 crystals of the latter. The orthoclase also encloses part of the quartz, 

 which in that case shows more or less perfect crystal outlines. The 

 iron-pyrites, often visible in hand-specimens of the rock, is a mineral 

 of secondary origin. 



(ii) Dark patch in the Shap Fell Granite. — These dark patches, 

 the ' heathen ' of the quarrymen, are very common in the Shap rock, 

 as in some others. They are not sharply marked off from the granite, 

 and often contain the same pink felspar crystals, showing that the 

 patches are not included fragments, but parts of the rock separated 

 out in concretionary fashion at an early stage of the consolidation. 

 They are invariably of finer texture than the granite, and of more 

 ' basic ' constitution. 



Under the microscope the slices show the same minerals as before, 

 but in different proportions. The biotite-flakes are more abundant, 

 though rather smaller : when cut nearly parallel to the basal cleavage 

 they show the hexagonal outline. Granular sphene is very plentiful, 

 besides magnetite, and numerous little colourless needles of apatite. 

 This last mineral, which is rarer in the proper granite, is the first 

 product of consolidation. Quartz is present as before, and felspar, 

 but now the oligoclase seems to preponderate over the orthoclase. 



Naturalist, 



