314 Prof. Sedgwick on the May Hill Sandstone , 



sections of North Wales. Again, between No. 3 (the sandstone, 

 &c. of Caradoc) and No. 4<a (the May Hill series), there is a 

 second break of continuity, to be supplied, perhaps, hereafter by 

 a considerable series of intermediate strata. With the conglo- 

 merates, &c. of No. 4a, commences a continuous series of de- 

 posits, representing the May Hill group and all the well-known 

 Silurian beds above it. 



We were unable, as before stated, to visit the Llandeilo 

 sections ; but I remember them sufficiently well to comprehend 

 the cause of their original misinterpretation in the 'Silurian 

 System.' 



The section (fig. 5) represents the sequence of deposits in 

 the Llandeilo country, as given by the author of ^he ' Silurian 

 System;' and, for several years after the publication of that 

 great work, it was accepted by geologists as a true and typical 

 sequence. Now, if the deposits represented by Nos. 2, 3, 4, 

 &c. be unbroken and continuous, it will follow, of perfect 

 necessity, that the neighbouring rocks of the Cambrian moun- 

 tains (No. 1) must be inferior to them all; for, on this hypo- 

 thesis, there is no place for their interpolation in any part of the 

 section immediately below the Wenlock shale, &c. (No. 4). 



In the hope of making this more clear, I give, in the accom- 

 panying ideal profile section (fig. 6), the general sequence and 



Fig. 6. 



S.E. Valley of the Towy. N.W. 



Ideal fault. 



In this section No. 1 represents the commencement of the mountains 

 which are coloured as Cambrian in the Silurian Map. 



No. 2 is a part of the saddle of Llandeilo flag as seen in the Vale of the 

 Towy. 



Nos. 3, 4, 5, 6 agree with the succession in the vertical section, fig. 6. 



position of the deposits in the neighbourhood of Llandeilo and 

 Carmarthen. In that typical country the Llandeilo flags exist 

 as a broken and very contorted saddle in the position of the 

 group (No. 2) in the Vale of the Towy. On the south-eastern 

 side of the contorted saddle, we find an apparently regular suc- 

 cession of deposits representing groups 3, 4, 5, &c. of the ideal 

 vertical section (fig. 5) ; but on the other side of the valley of 

 the Towy we have (towards the N.W.) a mountainous country, 

 composed of slaty and often contorted rocks, which were con- 

 sidered in the Silurian Map as inferior to the Llandeilo flag, and 

 to be represented by No. 1 in the ideal vertical section. 



