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INTRODUCTION. 



pushing the inquiry, and by his encouragement I was materially stimulated to do so. 

 In speaking of the labours of my friend, I may truly say, that he not only shed an en- 

 tirely new light on the crystalline arrangement or slaty cleavage of the North Welsh 

 mountains, but also overcame what to most men would have proved insurmountable 

 difficulties, in determining the order and relations of these very ancient strata amid 

 scenes of vast dislocation. He further made several traverses across the region in which 

 I was employed, and, sanctioning the arrangement I had adopted, he not only gave me 

 confidence in its accuracy, but enhanced the value of my work by enabling me to unite 

 it with his own ; and thus have our joint exertions led to a general view of the sequence 

 of the older fossiliferous deposits. 



In the mean time, some kind friends who had watched the progress of my labours, 

 and thought they might lead to important results, requested me to prepare a separate 

 treatise; and although I had been at first desirous of confining these views (as in the 

 case of every memoir I had previously written) to the Transactions of the Geological 

 Society, my objections were overcome by a very flattering requisition 1 . 



In obeying this call, I hope I shall, at all events, promulgate some new facts, and 

 place before geologists the history of a system of deposits, authenticated by numerous 

 good evidences in the shape of organic remains, by far the greater part of which have 

 never been published in any country. 



While maturing these views, I became convinced that, as this large and ancient group 

 contained peculiar organic remains, and was marked by distinctness of physical fea- 

 tures, lithological structure, and order of superposition, it was well entitled to be 

 considered a separate system. It was not therefore enough that, in my efforts to deve- 

 lope them, I had termed these deposits, in their natural descending order, the 

 "Ludlow," "Wenlock," " Caradoc," and "Llandeilo" formations; without some 

 collective name, no general view could be carried out, nor their relations to the 

 whole series of deposits established. For example, the group could not be defined 

 "Transition Rocks," because nearly every modern author had so extended the 

 meaning of this term, as to embrace in it all the deposits, from the carboniferous series 

 inclusive, down to the oldest slaty rocks in which organic remains cease to be percep- 

 tible ; whereas the object I had specially in view was, to point out the existence of in- 

 termediate rocks of great thickness, essentially different both in structure and organic re- 

 mains from the carboniferous strata. In early communications to the Geological Society, 

 adopting a provisional name, I called these rocks " fossiliferous greywacke " • but this 

 term was deemed objectionable, there being few beds in the group which can be recog- 

 nised as the " grauwacke" of German mineralogists — while there are fossiliferous 



1 Originating with the Right Honourable Frankland Lewis and his son Mr. George Lewis, this requisition 

 was soon subscribed by many resident nobility and gentry through the very friendly exertions of Lord Clive. 

 (See List of Subscribers.) 



