Abstract

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 micaceous; 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 Sciont.

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