2IO HARKER: NOTES ON NORTH OF ENGLAND ROCKS. 



and fan-like bundles, formed at the expense of the felspars and in 

 part of the biotite. 



Specimens from different parts of the large intrusive mass of 

 Ennerdale and Buttermere show various grades of the granophyric 

 structure, and form a very interesting study. This structure indicates 

 a simultaneous crystallisation of quartz and felspar during the final 

 stage of consolidation of the rock. 



(v) Minette from dyke in Knock Beck, near Appleby, Westmorland. — 

 The singular rocks known as mica-traps, belonging to the lamprophyre 

 family, are peculiarly susceptible to decomposition, and in ordinary 

 specimens contain a considerable proportion of carbonates of 

 secondary origin, causing the rock to effervesce briskly when acid is 

 dropped upon it. The present example shows a dull brown ground 

 spangled with flakes of brown mica. 



Under the microscope these flakes are seen as the most prominent 

 constituent, cut in various directions by the plane of the section. 

 The mica belongs to the biotite species, but the greater part of each 

 flake is partially bleached by discharge of the iron-oxide, only a 

 narrow border retaining the original vivid brown colour. The strong 

 dichroism of the mineral is seen best in this marginal zone ; using 

 the polarising Nicol only, we get an intense brown colour when the 

 length of the flake is parallel to the shorter diagonal of the Nicol, 

 and a pale brown for the perpendicular position. The strong cleavage 

 of the mica shows in a series of fine parallel lines, except when the 

 flake happens to be cut parallel to the cleavage-planes. A tendency 

 to parallelism is observable among different flakes in the same part 

 of the slide, and must be attributed to a certain flowing motion of 

 the mass after the formation of the mica. The smaller flakes belong, 

 perhaps, to a rather later stage of consolidation than the large ones. 



Magnetite is seen in the slice in opaque black sections, indicating 

 a very imperfect crystalline grouping. The only other original mineral 

 to be detected is represented by a few minute colourless prisms of 

 apatite. The felspar which must have formed the greater part of the 

 rock is entirely destroyed in the specimen examined, and there 

 remains only a dusty-looking ground-mass, composed, no doubt, of 

 minutely granular calcite, kaolin, etc., and interspersed with little 

 grains of clear quartz, which must be also a secondary product. 



(vi) Quartz- Minelle of Sale Fell, near Bassenthwaite, Cumberland. 

 — This rock is described as 'minette' by Mr. Clifton Ward, who in- 

 vestigated the distribution of its boulders. In some respects, however, 

 it differs from the usual type of Lake District minette, which is a 

 more normal example of the lamprophyre family. The Sale Fell 

 rock has less mica than these normal mica-traps, and quartz is an 



Naturalist, 



