194 



Mr; S. Allport and Prof. T. G. Bonney. [June 6, 



strata are seen always to strike towards the granite, and, in many 

 cases, to be cut off sharply by it. The line of junction is far from 

 being even or straight, for short dykes or masses of granite have been 

 intruded between the beds. 



The greater part of Mr. Allport's collection was obtained by follow- 

 ing the line of junction, and represents the altered condition of the 

 various beds through which the granite has broken. Hence it is 

 reasonable to suppose that the differences in mineral character which 

 they now exhibit are due to original differences in their constituents. 

 Another group of specimens represents the rocks at a greater distance 

 from any visible mass of granite. Of the former, Mr. Allport writes 

 that the specimens indicate the nature of the rock, from its actual 

 junction to a distance of about 12 yards from the granite, except in 

 two instances, where the nearest mass of the latter was visible about 

 40 yards away. Here, however, the actual distance very probably is 

 less, as the granitic mass slopes down beneath an intervening layer of 

 turf. 



Mr. Allport's collection contains some specimens from the granite 

 massif, but it has not been thought necessary to have them sliced for 

 microscopic examination. They show that, as might be expected, 

 the rock varies considerably in texture and somewhat in mineral com- 

 position. Some are granophyres more or less porphyritic, others true 

 granites. None are rich in quartz ; one, in which biotite is abundant 

 and a fragment of altered rock is included, is poor in this mineral. 

 One contains small wine-red garnets. The dominant tint is some 

 shade of grey. 



Among clastic rocks, confining ourselves to the practically non- 

 calcareous varieties, and putting aside the coarser breccias and con- 

 glomerates (none of which were seen by Mr. Allport in the district), 

 we find every gradation from the finest clays either to pure quartz 

 sandstones or to those miscellaneous grits which are often called 

 greywackes. This old-fashioned and rather vague term may be con- 

 veniently used for a group of rocks especially common among the 

 older Paleeozoic strata of Britain.* These greywackes consist largely 

 of quartz grains (often from 01" to 0'02" in diameter), with a fair 

 proportion of fragments of about the same size or occasionally rather 

 larger, among which the following may often be recognised: — 

 (a) Composite quartz grains, including fragments of vein quartz and 

 quartzite ; (6) fragments of felspar of various species ; (c) fragments 

 of granitoid rock ; (d) fragments of argillite and slate ; (e) fragments 

 of phyllite and fine-grained schists (generally mica-schist) ; (/) frag- 

 ments of volcanic rock, glassy, cryptocrystalline, or scoriaceous. 

 If one may generalise from a rather limited number of observations, 

 the more acid lavas predominate over the basic, and among these 

 * Bonney, Address to Section C of the British Association, 1886 (Birmingham). 



