Yol. 68.] GEOLOGICAL STRTJCTFRE OF CENTRAL WALES. 329 



is now known of the distribution of the great rock-groups to enable 

 us to indicate in broad outline the chief structural features of the 

 area. 



In the present communication the attempt is made to bring 

 i;ogether a summary of the results of various investigations which 

 Taear closely upon the rock-succession and structure of the area, and 

 to add various personal observations made at different times which 

 have not hitherto been recorded. On the basis of these combined 

 investigations a map has been prepared showing the general distri- 

 bution of certain rock-groups which are comparatively easy to 

 Tecognize, either by their lithology or by their organic contents. 



This map has been constructed without reference to the physical 

 interpretation of the region, but it is obvious at a glance that 

 certain structural features and their relation to the present topo- 

 graphy are brought out with great clearness. (See PI. XXXIV.) 



The detailed structure must, however, still remain a subject for 

 further research. There are other problems also which await 

 -solution ; some of these are indicated below. 



II. HrsTOEicAL Eeview. 



The series of slaty rocks, sandstones, and conglomerates which 

 -occurs to the west of the main Wenlock boundary was not closely 

 investigated by Murchison ; he remarks ^ that 



■'there is, indeed, no well-defined line of separation between the coarse 

 quartzose slates of Rha.yader and the masses of more crystalline slate which 

 are repeated upon parallel lines between that town and Plinlymmon. . . . They 

 all belong to the upper group of the Cambrian System, and are of the same 

 age as many mountains in Montgomeryshire (Moel-ben-tyrch, &c.), the whole 

 of which have been proved by Professor Sedgwick to be of younger date than 

 the slates of Merionethshire, &c.' 



It is clear that the structure of this tract was unknown to 

 Murchison, and on p. 360 (op. cit.) it is stated that the 



' separation of the Lower Silurian rocks from the Upper Cambrian has been 

 generally effected by assigning to the former those beds which contain fossils 

 and to the latter those which do not.' 



Sedgwick's connexion with the district was brief ; in 1846 he 



' made two hasty traverses through it between the Upper Silurian groups and 

 the sea — one from Aberystwyth to Builth, the other from Llandovery to 

 Aber Aeron by Llampeter, and thence back by the road from Llampeter to 

 Carmarthen.'^ 



He had previously traversed the same lines in 1832. As a result 

 of these visits he distinguished four principal groups among these 

 rocks, which he described as the System of South Wales and later 

 as Upper Cambrian (meaning thereby the strata between the Bala 

 Limestone and the base of the May Hill Sandstone, or what is now 

 generally known as the Upper Llandovery). 



1 ' Silurian System ' 1839, p. 317. 



2 Q. J. a.S. vol. iii (1847) p. 150. 



2b2 



