FROM Tin; CR0S8 FELL INLIER. 523 



sometimes shows luimeroiis very miiiuto needles (Vrutilc) arraii^cd 

 along the husal cleavaji;e, parallel to the sides otHhe hexagon [iilS]. 

 The most characteristic inclusions, however, are very minute 

 crystals, probably of zircon, which are invariably surrounded by 

 strongly pleochroic halos. These halos are very conspicuous, re- 

 producing in the pale interior of the flakes the dark colour of the 

 brown border. A common decomposition-product of the mica is 

 calcite, or possibly dolomite, often forming lenticles or plates along 

 cleavage-planes [445j *. 



In addition to mica, some at least of the rocks, such as those of 

 Dry Sike, have contained augite. The mineral is completely 

 destroyed, but the pseudomorphs show the characteristic octagonal 

 cross-section [915]. Original magnetite is not plentiful in our 

 specimens. Most of the slides contain little crystals of pyrites, 

 which moulds the biotite, and is perhaps a secondary mineral. The 

 porphyritic felspars, when recognizable, arc seen to be orthoclase 

 and a i)lagioclase with rather low extinction-angles ; but most of 

 the felspars are deeply altered, and some are entirely replaced by 

 calcite [9U]. 



The quartz-grains, already alluded to, present in section an 

 ov^oid outline, or, more rarel}'', rounded crystal-forms [915]. The 

 borderiiig ring is seen to consist of minute crystals of fibrous horn- 

 blende, pale green in a slice. These are partly moulded by the 

 quartz, while, on the other hand, they are not clistinctly separated 

 from the mass of the rock. The appearances suggest a reaction 

 between the quartz-grains and a corroding magma, and recall the 

 primary quartz-grains with a coating of augite-granules described 

 by Iddings t in certain basalts. 



The groundmass of these lamprophyric rocks is for the most part 

 too much decomj)osed for precise study. It has probably been 

 microcrystalliue throughout. In some cases the mica seems to 

 belong to two generations, of which the later, in smaller flakes, 

 forms part of the ground [445, 446 «]. The bulk of the ground 

 has consisted of felspars, which sometimes sank almost to microlithic 

 dimensions [915]. The most abundant decomposition-products are 

 calcite and quartz. Where these occur collected in distinct patches, 

 the quartz is often idiomorphic and moulded by the calcite. 



6. Basic Intrusive IIocks. 



The chief intrusive rocks of basic character in the district are the 

 diabases of Guns Fell. These show considerable variations in texture. 

 In the coarse-grained type it is easy to recognize the rectangular 

 crystals of striated felspar and the dark green cleavage-faces of 

 augite, or, again, black lustrous plates of hornblende. As will 

 appear, this last mineral is not an original constituent. In the 



* This is the slide figured in Teall's ' Brit. Petrogr.' pi, xxxii. fig. 2. 

 t Aiuer. Journ. Sci. (3) vol. xxxvi. (1888) p. 208; Eull. U.S. Geol. Siu-v. 

 < 181)0) No. ()G. 



