On some Nodular Felstones of the Lleyn

Publisher Website
Google Scholar
Abstract
I n the descriptions which have been given of various igneous rocks from the Lleyn, no details have, I believe, been published of the two small outcrops near Pwllheli, to which this paper refers. Some specimens, which I collected in 1885, I showed to Professor Bonney, and, at his suggestion, I tried to make a more thorough examination of the rocks. The two masses rise, like small islands, from the drift-covered plain around, and project seawards in the headlands of Pen-y-chain and Careg-y-defaid. They are marked on the Survey map as consisting of intrusive felspar porphyry with agate nodules. I propose, first, to describe the general types of rock, and, secondly, to notice certain structures, especially those connected with the nodules. I. G eneral S uccession A nd A ge O f T he R ocks . At Pen-y-chain the rocks show distinct evidence of stratification, and the beds seem, on the whole, to dip about N.N.E., although with irregularities, such as might be expected in volcanic masses. Thus the lowest strata which I reached were the rocks near the south or south-westerly point. They form the seaward part of the cliff, against which the long beach from Pwllheli terminates. The rock is darkly veined, with subangular fragments of whitish felstone, which shows faint indications of fluidal structure and contains porphyritic felspar and quartz. This rhyolitic rock seems to have undergone a brecciation in situ ; for some of the neighbouring fragments were evidently once continuous, and the blackish veins fade off at places and are lost in the felstone.

This publication has 0 references indexed in Scilit: