HARKER : NOTES ON NORTH OF ENGLAND ROCKS. 239 



imagine the minute filjres to be partly quartz, partly felspar. Such 

 spherulites show a more or less marked black cross when viewed 

 between crossed prisms. 



Both Mr. Clifton Ward and Mr. Teall have made the interesting 

 •observation that the granophyre appears to pass gradually into the 

 -quartz-bearing gabbro noted above. Specimens may be collected to 

 •show a mingling of the characters of the two very different types. 

 •One such, from a loose block in Caldew Beck, has been examined 



[419]- 



Some portions of this slide show the most perfect and typical 



•examples of micropegmatite together with grains of quartz and small 

 crystals of felspar. The quartz predominates, and the grains are 

 ■continuous with the quartzose element of the adjacent micropegmatite. 

 Elsewhere in the same slice we see the larger felspar crystals with 

 ■close tvvin-striation, the brown plates of diallage, the deeper brown 

 partly decomposed mica, the green hornblende, probably secondary, 

 the irregular magnetite grains, and the subordinate interstitial quartz 

 of the gabbro type. There is a complete gradation. Hexagonal 

 prisms of apatite occur throughout the slide, as do also irregular 

 .granules of light-brown highly refractive sphere. 



(ix) Spherulitic quartz-porphyry dyke at Greensides Mine, 

 Helvellyn. — This is a rock with reddish-brown ground-mass, enclosing 

 crystals of both quartz and felspar. In a thin section [461] it 

 exhibits a most beautiful illustration of the spherulitic structure. 

 The ground presents a confusedly crystalline aspect, the quartz and 

 felspar being only imperfectly individualised, though here and there 

 ■one or other mineral has separated, and collected into a patch large 

 ■enough to show its optical characters. The most striking feature, 

 however, consists in numerous little spherical growths having 

 .a marked radial structure, and giving a black cross when seen 

 between crossed Nicols. This last point is characteristic of 

 •spherulitic growths : where the structure is most typically developed, 

 the cross is quite distinct, and on rotating the stage its arms remain 

 fixed in direction, viz., parallel to the diagonals of the Nicol's prisms. 

 This is evident only in the more perfect spherulites in the rock in 

 question. The spherulites are partly free, partly attached to the 

 porphyritic crystals and especially to the quartz. The ground-mass 

 is further remarkable for containing innumerable little needle-shaped 

 crystallites, which sometimes show a tendency to parallelism of 

 position, and, in particular, lie parallel to the outline of any neigh- 

 bouring porphyritic crystal — a well-known result of a flowing 

 movement in the rock subsequent to the formation of both crystals 

 and crystallites. 



August 1S90. 



