304 Prof. Sedgwick on the May Hill Sa/idstone, 



the Bala limestone ; and if we make sections from these highest 

 beds to the limestones on the two sides of the trough, we find 

 a discrepancy in thickness which can be readily explained by the 

 discordant position of the overlying mass, and cannot, I think, 

 be explained any other way. I state this fact from my remem- 

 brance of the sections made by Mr. Salter and myself in 1843. 



Lastly, it is 1 think clear, from a glance over the map of the 

 Government Survey, that their so-called ''Middle Silurian '^ 

 group is generally unconformable to the older Cambrian rocks 

 on which it rests. It must, therefore, be obvious, that the task 

 I had proposed to myself was immediately connected with the 

 facts and conclusions of my preceding paper (read Nov. 3rd, 

 1852) ; but the bad state of my health compelled me, very 

 reluctantly, to abandon the greater part of it. 



There remained, however, a second and shorter task in which 

 Professor M'Coy had promised to join me, so soon as he had 

 completed his work in the Cambridge Museum. We proposed 

 to examine in detail the section of Mathyrafal, near Meifod, 

 the sections of the Pentamerus limestone on the flanks of the 

 Longmynd, the sections of the Onny and of Caer Caradoc, and 

 lastly, the sections of Builth, Llandovery, and Llandielo *. I 

 thought that the facts exhibited in these sections must have a 

 direct bearing on the conclusions we had drawn in the preceding 

 year. A very vexatious accident detained us full three weeks 

 beyond the time we had fixed on for the commencement of our 

 task. It was in consequence left incomplete, and our examina- 

 tion of the following sections was confined within the limits of a 

 single week, during more than half of which I was unable to 

 take the field, or to give any help to my fellow -labourer. 



I. Mathyrafal Section. 



This section is seen on the north side of the valley, a few 

 miles above Meifod, and its upper beds are now cut through by 

 the new road (from Meifod to Llanfair) which did not exist (in 

 1843) when the section was examined by Mr. Salter and myself. 

 I will not enter on details which have been already published in 

 the Journal of the Geological Society, but I wish to recall to 

 memory the fact, that there are at least two calcareous bands in 

 the lower part of this section ; and that the upper band, which 

 is concretionary and discontinuous, is associated with a coarse 

 conglomerate and with a hard sandstone, which appears in a 

 nearly vertical position by the side of the new road. These ver- 

 tical beds are overlaid by highly fossiliferous shales and flag- 

 stones of a considerable thickness, which gradually lose their 



* All the above are critical sections, at the junction of the groups I call 

 Cambrian, with the overlying groups which are undoubtedly Silurian. 



