134 J, E. MAEPv ON THE FOSSniFEB-OTTS 



17. Fossiliferous Cambrian Shales near Caernarvon. By J. E. Make, 

 Esq., St. John's College, Cambridge. (Read March 8, 1876.) 



(Communicated by Prof. T. Mc K. Hughes, F.G.S.) 



The shales under consideration extend from about three miles S.W. 

 of Caernarvon to Bangor, being roughly parallel to the Menai Straits. 

 They are faulted against Lower Cambrian to the east, and disappear 

 against a dyke on the west. They are, as a rule, greyish black to 

 bluish black in colour, most commonly tolerably sandy and mi- 

 caceous ; but in places, as where the fossils are found, they are chiefly 

 clayey with hardly any admixture of sand. 



The fossils were obtained from three places, all within a mile of 

 Caernarvon and on the banks of the Seiont. 



The first place is close to Pont Seiont, on the E. side of the river, 

 where an arrow marks the dip in the Ordnance map. The shales 

 here are bluish black, dipping at about 45° S.E., containing a large 

 amount of iron oxide, from the decomposition of iron pyrites, which 

 is itself found in nests and veins scattered through the mass of shales, 

 but especially towards the top of the section ; and the outer surfaces 

 of the rock weather to a rusty or olive-brown hue. 



The rock is of a splintery nature ; and it is rather difficult to get 

 large pieces. It has a concretionary structure all through, breaking 

 up into lenticular pieces ; and it is in the interior of these pieces that 

 most of the fossils occur. 



Most of the fossils came from the bottom of the section, and were 

 scattered somewhat capriciously, so that one might work for two or 

 three hours without finding a single specimen, and then suddenly 

 come upon two or three. Graptolites are the most abundant fossils 

 in this place. About 35 yards S.W. of the place from which the 

 fossils were obtained runs a greenstone dyke, parallel to the bedding 

 of the rock, and altering the shales to the distance of about four 

 yards from the edge of the dyke. The shale, where altered, has 

 a subconchoidal fracture, and a glazed porcellaneous appearance 

 with slickensided and iridescent joint-surfaces. One Graptolite oc- 

 curred here at the outer edge of the altered rock ; but unfortunately 

 it fell to pieces. On the other side of the dyke no rock is seen ex- 

 posed ; but about 50 yards further on another dyke is just visible, in 

 weathered bosses, at the side of the road. 



The second place where the fossils were found is on the S. W. side 

 of the river, and a few hundred yards nearer to the mouth of the 

 river than the preceding place. Here a series of sections is exhibited 

 along the old tramway from Caernarvon to Wantlle. The section 

 where the fossils were found is the third from the tramway bridge 

 over the river, and about opposite the letter " a " in the " road " 

 of " Wantlle railroad." The shale at this place is very similar to 

 that just described, but breaks up into larger masses. As the con- 

 cretionary structure is not so strongly developed, Graptolites were 



