Vol. 56.] SILUEIAN SEQUENCE OF EHATADER. 69 



of the Geological Survey maps of the Mid-Wales district that the 

 value of the graptolite as a geological index was realized. 1 



The monotony of the rocks, their folding and their cleavage, and 

 the rarity of fossils, have all conspired to render this region un- 

 attractive to the amateur geologist. Some reconnoitring surveys 

 have been made by three or four investigators, and fossils have been 

 quoted from one or two localities. But, as regards the detailed 

 geological structure of this great region, even at the present day 

 little more is known to the scientific world than at the time when 

 Murchison first attacked the country with the geological hammer. 



After his first survey (1839) Murchison placed the rocks of the 

 whole of this area in the Cambrian System. 2 Eight years later 

 (1 847) Sedgwick attempted abroad grouping of its strata, and founded 

 his well-known Aberystwyth and Plynlimmon Groups. 3 



About 1850, the Geological Survey maps of the district were 

 published, prepared from the surveys of Eamsay and Aveline, 

 and the greater part of the region was indicated as Lower Llan- 

 dovery, on the ground that the Survey officers had actually dis- 

 covered Lower Llandovery fossils within its limits. 4 



Nothing of apparent importance was brought forward during the 

 succeeding thirty years ; but in 1869 Mr. Hopkinson published a 

 list of a few graptolites which he had detected at Aberystwyth. 5 

 At that time, however, no great significance was attached to his 

 discovery. 



The first and, indeed, the last research-work, however, that made 

 any serious attempt to elucidate the structure of the region and 

 to prove the relative antiquity of the members of the complex was 

 that of the late Walter Keeping, who, in 1881, 6 showed that nearly 

 the whole of Central Wales was made up of rocks of Upper Llan- 

 dovery age, much folded and contorted. He suggested, at the 

 same time, that the Tarannon Shales were likely to be present 

 in certain areas. 



During the last three or four years I have had exceptional oppor- 

 tunities for studying the strata of a portion of this great rock- 

 complex of Central Wales. About 4 miles west of the town of 

 Khayader, the River Claerwen unites with the Elan, and to the 

 gathering-ground or common basin of these two rivers the city 

 of Birmingham has recently resorted (1892) for a new water- 

 supply. The scheme of the engineer, Mr. James Mansergh, 

 Y.P.I. C.E., involves the erection of masonry-dams for impounding 

 the waters of these streams. The dams are now in course of 

 construction, and the pipe-line, which crosses the Wye imme- 

 diately south of Rhayader, is now practically completed for some 

 12 miles to the eastward. Having been engaged as assistant- 



1 Quart. Journ. Geol. Soc. vol. xxxiy (1878) p. 240. 



2 ' Silurian System ' 1839, pp. 316, 317. 



3 Quart. Journ. Geol. Soc. vol. iii (1847) p. 153. 



4 Geol. Surv. Mem. vol. iii (1866) Ramsay's 'Geol. of North Wales" 1st ed. 

 p. 8. 



5 Journ. Quekett Micr. Club, vol. i (1869) p. 116 & pi. viii. 



6 Quart. Journ. Geol. Soc. vol. xxxvii (1881) p. 141. 



