Prof. W. Keeping — Geologij of Aherystunjth. 539 



gravity is constantly at work to take advantage of these sliiftings, 

 and to drag the slate or rock-bed over ; in the course of their 

 frequent loosenings they naturally come to topple over more or 

 less down the hill. 



Large slate-workings may be visited in the neighbourhood of 

 Corris, other good quarries are worked at Taren y Gesail and S.W. 

 of Machynllyth (Morbern Quarry), whilst numerous small openings 

 for local use are met with about Machynllyth, Tregaron, Strata 

 Florida, Goginan, and, indeed, throughout the country of the 

 Metalliferous Slate group. 



Metalliferous Veins. — A glance at the Geological Map of Cardigan- 

 shire will show that the majority of the mineral veins run more or 

 less parallel to one another in an E.N.E. and W.S.W. direction, 

 roughly speaking, and we may also distinguish a separate set 

 running more nearly from N.W. to S.E. So also, as we overlook 

 the mines themselves from a vantage point, we may often notice 

 that they are arranged in lines — those in the same line being probably 

 all workings in one vein.^ Descending into an ordinary mine, we 

 shall find that the vein is a vertical, or nearly vertical, wall-like 

 mass of mineral matter, sharply defined on both sides by the walls 

 of the vein, which are often found to be polished and striated. 

 The thickness of the " lode " or vein varies, but is very commonly 

 about four or six feet; lateral ofi'shoots are frequent. The vein 

 consists of vein rubbish, spars, and ore. The " vein rubbish " is 

 mostly hard slate-rock, like the country rock around ; the spars are 

 almost exclusively quartz, and the ores are usually lead (lead 

 sulphide, Pb S) zinc, or black jack (Zn S) or copi^er (copper 

 pyrites Cuj S, Fe., S^). Silver is found in valuable quantity, but it 

 is not distinguishable in the vein of itself, being mixed with the 

 lead in small proportions. Practically, however, its presence is 

 detected in the rough state, for the •' silver lead " is commonly a 

 paler and more finely crystalline ore than that in which silver is 

 scant5^ 



It is probable that these veins are in the lines of old faults or 

 rock dislocations, by which open fissures were produced, and after- 

 terwards the cavity was filled in with spars and rubbish from the 

 sides and "country rock" around, and the ores were brought by 

 sublimation from below. 



The Records of the Ancient Glaciers. — We must now leave the solid 

 geology of the country to notice what is the nature of those newer 

 formations which are commonly classified under the heading of 

 " surface deposits." And here the relics of the Great Glacial Period 

 first claim our attention, since they form the oldest of this surface 

 group, and also because they are so very conspicuous. 



We need only go to Aberystwyth Castle Ground, and pick up the 

 stones on the cliff over the railings, to find at once those peculiar 



1 It is remarkable how very frequently a lead mine is found at the head of a 

 valley, and again they are frequent along the sides of valleys. This is probably due 

 to the metalliferous veins, being lines of comparative weakness, so determining the 

 lines of action of subaerial denuding forces as to produce vaUeys. 



