488 



Prof. Sedgwick on the May Hill Sandstone, 



wish to expunge No. 3 (the Caradoc sandstone) from the section, 

 and to place the Llandeilo flag immediately under the Wenlock 

 shale. The hypothetical interpolation of the Caradoc sandstone 

 did not, however, at all affect the general inference I drew from 

 the sketch. It conveys, however, such an inadequate notion of 

 the thickness of the nearly vertical beds which overlie the north 

 side of the Llandeilo saddle, that I will endeavour to give a better 

 ideal representation of it. 



Fig. 5. 

 N. and S. section through the valley of the Towy below Llangadoc. 



1. Old Red Sandstone, 

 la. Tilestone. 



2. Wenlock and Ludlow. 



3. Contorted Llandeilo saddle. 



Hiehly inclined slate rocks, &c. of very great 



thickness. 

 Commencement of the great undulating 



groups. 



If the above sketch convey anything resembling a correct 

 notion of the relations of the great mineral masses, it must 

 inevitably follow that the Llandeilo flag is no true base to the 

 overlying Silurian terrace, and that it has no immediate relation 

 to the Wenlock shale except that of an accidental juxtaposition 

 arising from an irregular overlap of the true Silurian groups ; 

 on the contrary, that it is deeply imbedded in the great Upper 

 Cambrian group, which rolls, in many times repeated and irre- 

 gular undulations, to the coast of Cardigan Bay ; or, if it be 

 crossed on a more northern line, which rolls in like undulations 

 till it is lifted up and arrested near the south-eastern flank of 

 Cader Idris. I call the group Upper Cambrian, because, however 

 different in mineral type, it is the representative of the great Bala 

 group (No. 3) of the Tabular View (supra, p. 362)*. 



* I once, with a view of bringing the older rocks of Wales into com- 

 parison with those of Cumberland, limited the Upper Cambrian group to 

 the rocks which commence with the Bala limestone and end with (what I 

 now call) the May Hill sandstone. The change I afterwards made (in 

 drawing the Upper Cambrian base hne on the S.E. flank of Cader Idris or 

 the E. flank of great Arenig) involved no change of principle; it was a 

 mere shifting of a line of demarcation among the subdivisious of an un- 

 broken series. Whether the calcareous beds of the Llandeilo group be, or 

 be not, the exact equivalents of the Bala Umestone I do not pretend to 

 determine. 



The May Hill sandstone, which in the typical Silurian country is of 

 great thickness and continuity, gradually dies away and disappears in the 

 country described in this paper. Hence it sometimes appears and some- 

 times disappears from the base of the overlying Silurian groups ; and in 

 the latter case, the Wenlock shale forms the base of the Siliuian groups 



