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WALTER KEEPING ON THE 



and crystals of felspar, sometimes perfect but more often fragmen- 

 tary, very numerous and conspicuous. These constituents are held 

 together in the dark argillaceous greywacke matrix. The rock often 

 closely simulates a volcanic ash*. 



In thin sections, examined under the microscope, the Pen-Craig 

 rock exhibits irregular angular and subangular fragments of quartz 

 and felspar, the interstices filled with the dark, opaque (argillaceous) 

 matrix. The quartz is somewhat cracked, and includes numerous 

 cavities and minute spicular microliths; these latter are grouped 

 into wavy stratoid zones, between which the principal cracking of 

 the quartz runs. The felspar is mostly in the form of very angular 

 crystal-fragments, usually much decomposed, and of powdery ap- 

 pearance ; but some of the better-preserved fragments show the 

 characteristic ribbon banding of the Plagioclase group when ex- 

 amined with polarized light. 



The ordinary finer-grained beds are usually of darker colour than 

 these, and contain less felspar ; they are, however, for the most part 

 quite similarly constituted, the felspar crystals often being readily 

 recognizable when decomposed into irony spots over the weathered 

 portions of the rock. Paler thin bauds,' very compact in texture, 

 occur in the southern part of the district around Llangrannog. 



The presence of crystals of iron-pyrites occasionally gives a marked 

 feature to the grits ; and at Pen Craig, Llanilar, and in the Garthen 

 valley, Eglwys Pach, some beds occur beautifully studded with these 

 brassy cubes. 



The ordinary thickness (4-6 inches) of the beds is very constant 

 throughout the area of the Aberstwyth grits ; but more massive beds 

 (1-1 1 foot) occur under Allt wen and at Aberaeron, in the Garthen 

 valley, and at Llangwyryfon (2 J feet). At Cefn Coch, Pen Pegwyns, 

 and along the coast south of Aberystwyth still thicker beds are seen 

 (3-4 feet) ; they are worked for building-stones. At the southern 

 limit of the series, near Llangrannog, the grits become thin-, irregular, 

 and inconstant, thus gradually dying away, to be replaced by the ar- 

 gillaceous slate-rocks. 



Two structural peculiarities are very characteristic of the Car- 

 diganshire grits — namely, the remarkable contorted lamination seen 

 on a fractured surface, and an abundance of fucoid markings, to- 

 gether with strange-looking ripplings, ridgings, volutings, and other 

 raised forms of structure found upon the undersurfaces of the grit- 

 beds f. The rock is jointed, sometimes into large blocks good for 

 building- and flag- stones, but often much more closely into regular 

 oblong or rhomboidal fragments. At Allt-wen cliff and elsewhere 

 it is divided up into oblong, prismatic, or rudely lozenge-shaped 

 fragments a foot long, forming a kind of coarse grit-rab. 



* This abundance of felspar crystals, so general in the Silurian rocks (Upper 

 Silurian of North Wales, South Wales, and the Lake-district), points to the 

 neighbouring presence of a vast mass of early, perhaps primaiTal, igneous 

 rocks as the great source of sediment-supply in Silurian times. 



t It is proposed to give a more particular account of the rock-structure of 

 Cardiganshire in a separate paper. 



