"642 EEPOBT— 1891. 



cannot he a doubt that the post-Carboniferous earth-movements which so power- 

 fully affected and folded the beds in North Devon extended into and produced 

 almost identical results in South Pembrokeshire. In the latter area, however, the 

 succession remains clearer, and can be traced more continuously. 



The base of the Silurian (Upper Silurian of Survey) is exposed at many 

 points, and the lower beds, usually conglomerates, repose transgressively on the 

 Ordovician, and even on some pre-Cambrian rocks. Near Johnston and Stoney 

 Slade the conglomerate contains numerous pebbles of the Johnston and Great Hill 

 granite as well as of other igneous masses which were formerly supposed to be 

 intrusive in these beds. From the Silurian conglomerate to the Carboniferous beds 

 there does not appear to be any evidence of a very marked break in the series ; more- 

 over, all these beds were folded together and suffered equally by the movements 

 which affected the area. The axes of the folds strike from about W.N.W. to 

 E.S.E. The movements, therefore, at this time were in a nearly opposite direction 

 to those which affected the Ordovician and Cambrian rocks at the close of the 

 Ordovician period. Within the broken anticlinal folds portions of the old land 

 surfaces have been exposed in several places by denudation. 



The succession exposed in this area and the effects produced by the earth-move- 

 ments so nearly resemble those already described by the author as occurring in 

 North Devon, that he is convinced that the beds must have been deposited con- 

 temporaneously in one continuous subsiding area, and that the differences recognis- 

 able are chiefly in the basal beds, which were deposited on an uneven land surface. 

 He believes that the Morte slates of North Devon are a portion of an old land 

 surface on which the so-called Devonian rocks were deposited, and he also believes 

 that the Devonian rocks are only the representatives in Devonshire of the Lower 

 Carboniferous, Old Ked Sandstone (and possibly of some of the Silurian rocks), of 

 Pembrokeshire. A critical examination of the fossil evidence tends strongly to con- 

 firm this view. 



5. Vulcanicity in Loiver Devonian Roclcs. The Pratule Prohlem. 

 ByW. A. E. UssHEE, F.G.S. 



[Communicated by permission of the Director-General of the Geological Survey.] 



In the area extending south from the Middle Devonian volcanic series of 

 Ashprington to the Prawle there appears to be no proof of the occurrence of strata 

 older than Lower Devonian. There is no adequate reason for assuming that Lower 

 Devonian rocks as old as the Gedinnian occur on the surface, and there is no 

 certainty that the lowest beds are older than the Lower Coblenzian. 



The occurrence of local volcanic action in Lower Devonian time is proved 

 by a series of diabases and tuffs near Dartmouth, in the Kingswear Promontory, 

 near Stoke Fleming, and in the line of country west from Torcross. 



In association with the northern chloritic band (running from the mouth of 

 the valley on the north of Hall Sands on the east to Hope on the west) we find 

 volcanic materials identical in character with varieties of volcanic rocks associated 

 with the Devonian slates in the line of country west from Torcross ; and here and 

 there in the line of country west from Torcross the volcanic rocks assume a more 

 or less pronounced chloritic aspect. The junction of the slates on the north with 

 the northern chloritic band is a strictly normal one, the chloritic rocks being 

 almost invariably separated from the slates by brown volcanic materials which are 

 everywhere succeeded by the same type of Devonian slate, and in the Southpool 

 Creek and many other sections are found to pass insensibly into the chloritic type. 

 In the Southpool Creek section a hard bluish diabase (? aphanite) occurs in the 

 chloritic band. In the southern chloritic districts of the Prawle the volcanic rocks 

 maj' still be here and there detected by texture or colour. Volcanic rocks occur in 

 the mica schists of the Start coast, and can be detected even when only a few 

 inches in thickness. At Spirit-of-the-Ocean Cove chloritic rock with much calc- 

 spar occurs in association with tuffs and a grey rock with incipient foliation, pre- 

 senting a slightly gneissoid appearance, and apparently a much sheared diabase. 

 The association of the chloritic rocks with the mica schists is of as intimate a 



