140 Reports and Proceedings — 



deposited against a bank or sloping surface of Wenlock shale ; and 

 they state that the great majority of the pebbles in the conglomerate 

 are quite unlike any rock known in the district, but closely resemble 

 the Upper-Ludlow beds of Kendal and Central Wales. The authors 

 discuss the origin of the pebbles, and suggest " the probable exten- 

 sion of the Ludlow beds under Lancashire as the most likely source 

 from which they can have been derived." 



2. " On a New Group of Pre-Cambrian Eocks (the Arvonian) in 

 Pembrokeshire." By Henry Hicks, M.D., F.G.S. With an Ap- 

 pendix on their Microscopic Structure by T. Davies, Esq., F.G.S. 



In some new areas of Pre-Cambrian rocks discovered by the 

 author last summer in Pembrokeshire, some rocks of a character 

 hitherto unrecognized in this country were made out. As they were 

 found to hold there, and subsequently also in other areas, a very 

 definite stratigraphical position, with a vertical thickness of several 

 thousand feet, they have been separated by the author from the 

 other Pre-Cambrian groups under the distinctive name of Arvonian. 

 They were also found to occupy an intermediate position between 

 the Dimetian and Pebidian formations, and at all points, so far as 

 could be made out, appeared to be separated from each of those 

 formations by stratigraphical breaks. The new areas where they 

 are chiefly exposed are situated some few miles to the north of 

 Haverfordwest, where they form ridges running in a direction from 

 N.E. to S.W. They occupy an average width of about a mile, at- 

 tain at some points to a height of nearly 600 feet, and together 

 have a length of over nine miles. The rocks are flanked by Pebi- 

 dian and Cambrian beds along their N.W. borders, and on the S.E. 

 Silurian rocks have been brought against them by faults. In general 

 appearance, as well as in their more minute lithological characters, 

 they are easily distinguished from any of the rocks hitherto de- 

 scribed by the author as characteristic of the Dimetian and Pebidian 

 groups in Pembrokeshire. They are, however, so closely allied to 

 some of the true "halleflinta" rocks of Sweden, that it seems to the 

 author and Mr. Davies that this is the name that should be applied 

 to them in a petrological sense. In external aspect and in their 

 splintery fracture they resemble a hornstone. Under the microscope 

 they are seen to consist mainly of a crypto-crystalline ground-mass, 

 which, when examined with a high objective, is resolved into grains 

 of quartz, with an interstitial ingredient having but little action 

 on polarized light, but which presumably is felsite. There are also 

 numerous nests and fissure-like groups of quartz-grains disseminated 

 throughout, and sometimes angular fragments, distinct in size and 

 shape, are enclosed. These nests and fissure-like groupings are 

 frequently encircled also with bands of fibrous, chalcedony, the 

 structure of which is well exhibited with polarized light, and a 

 rude parallelism, suggestive either of an incipient foliation or of 

 stratification, is thereby given to the rock. The author and Mr. 

 Davies believe the origin of the rock to have been a sedimentary one. 



3. "On 'the Pre-Cambrian (Dimetian, Arvonian, and Pebidian) 

 Eocks of Caernarvonshire and Anglesey." By Henry Hicks, M.D., 



