350 CAMBRIAN AND SILURIAN IN EUROPE. [XV. 



zoic fauna; 3. The history of the lower palaeozoic rocks of 

 North America. 



I. SILURIAN AND UPPER CAMBRIAN IN GREAT BRITAIN. 



Less than forty years since, the various uncrystalline sedi- 

 mentary rocks beneath the coal-formation in Great Britain and 

 in continental Europe were classed together under the common 

 name of graywacke or grauwacke, a term adopted hy geologists 

 from German miners, and originally applied to sandstones and 

 other coarse sedimentary deposits, hut extended so as to include 

 associated argillites and limestones. Some progress had been 

 made in the study of this great Graywacke formation, as it 

 was called, and organic remains had been described from vari- 

 ous parts of it ; but to two British geologists was reserved the 

 honor of bringing order out of this hitherto confused group of 

 strata, and establishing on stratigraphical and palyeontological 

 grounds a succession and a geological nomenclature. The 

 work of these two investigators was begun independently and 

 simultaneously in different parts of Great Britain. In 1831 

 and 1832, Sedgwick, aided in the early part of his labors by 

 Mr. Charles Darwin, made a careful section of the rocks of 

 North Wales from the Menai Strait across the range of Snow- 

 don to the Berwyn hills, thus traversing in a southeastern di- 

 rection Caernarvon, Denbigh, and Merionethshire. Already, he 

 tells us, he had in 1831 made out the relations of the Bangor 

 group (including the Llanberris slates and the overlying Har- 

 lech grits), and showed that the fossiliferous strata of Snowdon 

 occupy a synclinal, and are stratigraphically several thousand 

 feet above the horizon of the latter. Following up this investi- 

 gation in 1832, he established the great Merioneth anticlinal, 

 which brings up the lower rocks on the southeast side of Snow- 

 don, and is the key to the structure of North Wales. From 

 these, as a base, he constructed a section along the line already 

 indicated, over Great Arenig to the Bala limestone, the whole 

 forming an ascending series of enormous thickness. This 

 limestone in the Berwyn hills is overlaid by many thousand 

 feet of strata as we proceed eastward along the line of section, 



