- HISTORY OP GRITS AXD SAXDSTOXES. 



7 



Composition and Structure. 



Cambrian.— The well-known Barmouth Grits of Xorth "Wales, 

 which occupy a large extent of country lying between Barmouth 

 and Harlech, are usually of a greenish-grey colour. They are ex- 

 tremely hard, and often enclose angular fragments of quartz above 

 a quarter of an inch in diameter. Sometimes these grits become 

 fine-grained, and assume a purple tint ; they are intermingled with 

 occasional bands of greyish-green and bluish slates, which, especially 

 towards the lower part of the series, attain considerable develop- 

 ment. Of these rocks Professor Ramsay remarks : — " The beds seem 

 to have been formed principally by the direct waste of rocks of a 

 granitic character, or at least into the composition of which crystal- 

 line quartz and felspar largely enter ° ; *. 



When a thin section of this rock is examined under the micro- 

 scope, it is seen to consist, mainly, of an aggregation of fragmentary 

 quartz and felspar united by a siliceous cement, which is throughout 

 permeated by a moss-like greenish mineral, a portion of which is 

 probably chlorite. The larger pieces of quartz and felspar are often 

 distinctly rounded, although the}* also sometimes present irregular 

 and perfectly sharp outlines. Pig. 1, PI. L, drawn by Mr. F. Rutley, 

 represents, in black and white, a section of this rock as seen in 

 polarized light, magnified 18 diameters, and containing much felspar. 



The quartz occasionally contains liquid-cavities enclosing moving 

 bubbles, but these are by no means numerous ; the greenish mineral 

 of the cement sometimes penetrates into fissures in the siliceous 

 grains. Two distinct species of felspar are present in considerable 

 quantities, the larger grains being, for the most pare, somewhat 

 rounded fragments, which, after having assumed the form of 

 pebbles, have sometimes been broken across their smaller dia- 

 meter, thus presenting one angular and one rounded termination. 

 j[he orthoclase is not much altered ; and a triclinic felspar, which, 

 from the optical properties it exhibits, is probably oligoclase, shows 

 brilliant lines of twinning when seen in polarized light. Some 

 of the quartz encloses hair-like crystals of rutile, while calcite, 

 magnetite, iron pyrites, and a few imperfect garnets are present in 

 the cementing siliceous base. An analysis of this rock is given, 

 page 21. 



The grits in the neighbourhood of Harlech are usually finer in 

 grain than the foregoing, but otherwise differ from it only in con- 

 taining a few imperfect crystals of epidote. 



Silurian. — Stiper Stone, from the neighbourhood of Shrewsbury, is 

 a fine-grained and exceedingly hard sandstone, the grains of which 

 are so closely cemented together by crystalline silica as to form a 

 quartzite. Many of the fragments of quartz, of which this rock 

 is mainly composed, are somewhat cloudy and are considerably 

 rounded, while others are colourless and transparent, with but few 

 fluid-cavities, which are, for the most part, full. The average dia- 

 meter of the grains is about inch, and some of the most pebble- 



* Geology of North Wales, p. 17, ed. 1. 



