292 Geological Society: — 



Dingle, and especially of one containing numerous red- and green- 

 sandstone pebbles, which enclose fossils of Upper-Ludlow forms, 

 and lying above the so-called " Bastard Limestone." From the 

 arrangement of the beds the authors believe that they may have 

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

 and they state that the great majority of the pebbles in the con- 

 glomerate are quite unlike any rock known in the district, but 

 elosely resemble the Upper-Ludlow beds of Kendal and Central 

 Wales. The authors discuss the origin of the pebbles, and suggest 

 " the probable extension 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., E.G.S. With an Ap- 

 pendix on their Microscopic Structure, by T. Davies, Esq., E.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 Arvo- 

 nian. 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, 

 attain 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 insterstitial 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 a fibrous chalcedony, the 

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

 rude parallelism, suggestive either of an incipient foliation or of 



