vi Sedgwick and Murckison 259 



to their true origin by Sedgwick, who thus made an 

 important contribution to the progress of volcanic 

 geology. 



By a curious coincidence, Sedgwick and Murchison 

 broke ground in Wales during the same summer of 1831. 

 But while Murchison determined to work his way down- 

 ward, from the known horizons of the Old Eed Sandstone 

 of South Wales into the greywacke below, Sedgwick, with 

 characteristic dash, made straight for the highest, ruggedest 

 and most complicated tract of North Wales. Eeturning 

 to the same ground the following year, he plunged into 

 the intricacies of the older Palseozoic rocks, and succeeded 

 in disentangling their structure, tracing out their flexures 

 and dislocations, and ascertaining the general sequence of 

 their principal subdivisions. It was a splendid achieve- 

 ment, which probably no other man in England at that 

 time could have accomplished. 



But valuable as this work was, as a contribution to the 

 elucidation of the tectonic geology of a part of Britain, 

 it had not yet acquired importance in general stratigraphy. 

 In the first place, Sedgwick's groups of strata were mere 

 lithological aggregates. They possessed as yet no distinc- 

 tive characters that would allow of their being adopted 

 in the interpretation of other countries, or even of 

 distant parts of Britain. They contained fossils, but these 

 had not been made use of in defining the subdivisions. 

 There was thus neither a basis for comparison with other 

 regions, nor for the ascertainment of the true position of 

 the North Welsh rocks in the great territory of Greywacke. 

 In the second place, there was no clue to the connection 

 of these rocks with any known formation, for they were 



