68 me. h. lapworth on the [Feb. I900 r 



I. Introduction. 



(1) The Ordovician and Silurian Complex of Central Wales. 



If an examination be made of any general geological map of 

 Wales, in which the outcrops of the various formations are marked 

 with the colours habitually used by the Geological Survey, the 

 attention is certain to be arrested by the broad spread of pale 

 pink or purple colour which occupies the central portions of the 

 map. It will be seen that this tint spreads over an area repre- 

 senting some 1800 square miles, or nearly one-fourth of the area 

 of the Principality: stretching from Machynlleth to Caermarthen, 

 and from the shores of Cardigan Bay nearly to the town of Builth. 



Round this unbroken field of pale tint upon the map sweep more 

 strongly-coloured bands which mark the outcrops of the bordering 

 formations; but within the central field itself little or nothing 

 is given which would enable one to gather an idea of the sequence 

 or distribution of its rocks. Even upon the latest Index Maps of 

 the Geological Survey, we find merely a few scattered reference- 

 letters, — b 2 , b 3 , b 4 , V, — which indicate that within this great region 

 rocks are known to occur belonging to the Llandeilo, Caradoc, and 

 Llandovery formations ; but no attempt is made to mark out the 

 boundaries of the various groups. 



The reasons are not very far to seek : we need only refer to any 

 published description of the rocks themselves. In the words of 

 the late Walter Keeping, ' Central and AVest Central Wales is made 

 up almost entirely of a great series of imperfect slates and grey- 

 wackes, . . . pale slates and grits. . . . The rock-beds are 

 astonishingly folded into violent contortions, with frequent inversions, 

 ... so as often to produce the misleading appearance of a regular 

 and continuous ascending series.' 1 



Indeed a glance at almost any of the Geological Survey maps of 

 the region, upon the scale of 1 inch to the mile, is sufficient to 

 convince a geologist of the great amount of rock-folding that has 

 taken place. Patches of conglomerates and grits are shown dotted 

 in yellow, in long lenticular strips amid a general mass of slates 

 in which the dip and strike remain nearly constant. This is an 

 almost certain indication that we are dealing with a country where 

 the rocks are so folded that the outcrops of the more conspicuous 

 strata do not mark continuous bands, but represent the crests of 

 denuded anticlines, or the laps of sharp inversions, as in the case of 

 the strata of the Southern Uplands of Scotland. 



Not only do these strata of Central Wales resemble those of 

 Southern Scotland in their monotony and in their excessive folding, 

 but they are similar in a third respect. They are almost everywhere 

 destitute of organic remains, with the exception of a few grap- 

 tolites ; and even these are difficult of extraction, owing to the 

 prevalent cleavage of the rocks. Further, it must be distinctly 

 borne in mind that it was not until thirty years after the publication 



1 Quart. Journ. Geol, Soc. vol. x^xvii (1881) p. 169. 



