240 HARKER : NOTES ON NORTH OF ENGLAND ROCKS. 



The porphyritic quartz, as well as the felspar, shows good crystal 

 outlines, viz., those belonging to the hexagonal pyramid cut in 

 various directions. These quartz crystals have irregular cavities 

 communicating with the exterior, and filled by material similar to the 

 ground-mass. The felspars are much decomposed. A few flakes of 

 partly destroyed biotite and little rounded crystals of pinkish garnet 

 complete the list of constituents. The latter mineral is easily 

 recognised by its isotropic nature, which causes it to remain dark 

 between crossed Nicols, and its high refractive index, which makes 

 it appear to stand out from the other minerals in the slice. It is 

 seen as little crystals of a deep-red colour in the hand-specimens. 



The Greensides Dyke is very similar to a neighbouring one on 

 Armboth Fell, which has been noticed by Clifton Ward and others, 

 and a slice [756] of the latter rock presents the same general 

 characters as the foregoing. 



(x) Micrografiite of St. Joh/i's Vale, Cumberland. — One of the most 

 interesting points connected with the acid irruptive rocks is the 

 variety of structural gradations which they exhibit between the plainly 

 crystalline type of the granites on the one hand, and the glassy 

 texture of some of the rhyolitic lavas on the other. Two main lines 

 of transition may be recognised in the character of the ground-mass 

 as seen under the microscope. One of these is through the 

 granophyric varieties such as the micropegmatite of Buttermere 

 and the spherulitic quartz-porphyry of Greensides and Armboth. 

 A complete series of intermediate varieties of structure might be 

 collected to fill the gap between the holocrystalline and the vitreous 

 types. Another line of transition is furnished by the granite- 

 porphyries, microgranites, etc., and consists in a simple diminution 

 m the size of the constituent grains of the rock, the several 

 constituents showing no sign of intergrowth, but remaining distinct 

 so long as they are visible. PorjDhyritic elements may be, and 

 usually are, developed in the varieties of both series. 



An excellent example of a microgranite is the rock which 

 constitutes several connected intrusive masses in the lower part of 

 the Vale of St. John. It has to the eye a compact appearance and 

 usually a grey colour, the porphyritic crystals of felspar being very 

 small. Sections [15 and 460] show that these crystals consist 

 exclusively of a plagioclase variety with fine twin-striation and narrow 

 extinction-angles. The crystals are altered almost to opaqueness,, 

 and show rectangular outlines. Flakes of biotite are present in 

 varying quantity ; sometimes bleached with separation of magnetite, 

 which remains as little granules between the cleavage-lamina of the 

 mica ; sometimes converted into a bright-green chloritic substance, 



Naturalist, 



