80 HISTORICAL PALEONTOLOGY. 



mountains, girded by precipitous cliffs. Industrially, the Cam- 

 brian Rocks are of interest, if only for the reason that the 

 celebrated Welsh slates of Llanberis are derived from highly- 

 cleaved beds of this age. Taken as a whole, the Cambrian 

 formation is essentially composed of arenaceous and muddy 

 sediments, the latter being sometimes red, but more commonly 

 nearly black in color. It has often been supposed that the Cam- 

 brians are a deep-sea deposit, and that we may thus account 

 for the few fossils contained in them ; but the paucity of fossils 

 is to a large extent imaginary, and some of the Lower Cambrian 

 beds of the Longmynd Hills would appear to have been laid 

 down in shallow water, as they exhibit rain-prints, sun-cracks, 

 and ripple-marks incontrovertible evidence of their having 

 been a shore-deposit. The occurrence of innumerable worm- 

 tracks and burrows in many Cambrian strata is also a proof of 

 shallow-water conditions ; and the general absence of limestones, 

 coupled with the coarse mechanical nature of many of the sedi- 

 ments of the Lower Cambrian, may be taken as pointing in the 

 same direction. 



The life of the Cambrian, though not so rich as in the suc- 

 ceeding Silurian period, nevertheless consists of representa- 

 tives of most of the great classes of invertebrate animals. The 

 coarse sandy deposits of the formation, which abound more 

 particularly toward its lower part, naturally are to a large 

 extent barren of fossils; but the muddy sediments, when not 

 too highly cleaved, and especially towards the summit of the 

 group, are replete with organic remains. This is also the case, 

 in many localities at any rate, with finer beds of the Potsdam 

 Sandstone in America. Limestones are known to occur in 

 only a few areas (chiefly in America), and this may account for 

 the apparent total absence of corals. It is, however, interest- 

 ing to note that, with this exception, almost all the other lead- 

 ing groups of Invertebrates are known to have come into 

 existence during the Cambrian period. 



Of the land-surfaces of the Cambrian period we know 

 nothing; and there is, therefore, nothing surprising in the fact 

 that our acquaintance with the Cambrian vegetation is confined 

 to some marine plants or sea-weeds, often of a very obscure and 

 problematical nature. The " Fucoidal Sandstone " of Sweden, 

 and the " Potsdam Sandstone " of North America, have both 

 yielded numerous remains which have been regarded as mark- 

 ings left by sea-weeds or " Fucoids ; " but these are highly enig- 

 matical in their characters, and would, in many instances, seem 



