238 HARKER : NOTES ON NORTH OF ENGLAND ROCKS. 



hexagonal prisms of apatite and rather irregular grains of magnetite. 

 The last mineral formed is quartz, with a dusty appearance owing to 

 a multitude of minute inclusions. This mineral — an uncommon one 

 in gabbros — is wedged into the interspaces left between the earlier 

 constituents. 



The rock of White Crags was analysed by Mr. J. Hughes. 

 His results seem to indicate about 50 or 51 per cent, of a labradorite- 

 bytownite felspar, 28 of the pyroxenes, which cannot contain much 

 alumina, 20 of quartz, and one or two per cent, of magnetite and 

 apatite. 



A slide [79] from the last-named locality shows the same general 

 characteristics as before, except that the hypersthene is almost 

 wanting. We see, however, a quantity of green hornblende with its 

 characteristic cleavage, pleochroism, and low extinction-angle. Some 

 of this is so associated with the brown diallage as to show that it has 

 been derived from the alteration of the latter mineral, and this is 

 probably the origin of all the hornblende in the rock. 



(viii) Granophyre of Carrock Fell. — This rock, exposed on the 

 upper part of Carrock Fell itself, is reddish to brownish-grey in 

 colour, with minute porphyritic felspars and little greenish spots. 



Under the microscope [890] we see that, with the exception of 

 the little porphyritic felspar crystals, the rock consists of a ground- 

 mass of felspar and quartz, the structure showing variations between 

 certain limits. In places there is a finely granular texture, giving 

 the ' microgranite' of some petrologists ; but the most common type 

 is the micropegmatitic, produced by a minute intergrowth of felspar 

 and quartz, each mineral having a definite crystalline orientation 

 over a considerable area in the slide, as is proved by rotating the 

 stage between crossed Nicols, when the quartz over a large part of 

 the field is found to be dark in one position, and similarly the felspar 

 in another position. In natural light the clearness of the quartz and 

 the turbidity of the felspar, which is partially decomposed, distinguish 

 the two elements very clearly. Frequently, it may be noticed that 

 the micropegmatite growth has grouped itself about the porphyritic 

 crystals, in which case the felspar of the micropegmatite is proved 

 (by its simultaneous extinction) to be in crystalline continuity with 

 the crystal which has served as a nucleus. In other places there is 

 a rather ' centric ' arrangement of the intergrowth, independent of 

 any nucleus. This is seen sometimes when the micropegmatite is on 

 an excessively minute scale. From this, it is not a long step to the 

 ' spherulitic ' structure observable in some other specimens, in which 

 the quartz and felspar are only imperfectly individualised, and the 

 general eftect is that of a radiate fibrous growth in which we may 



Naturalist, 



