8O HISTORICAL PALAEONTOLOGY. 



muddy sediments, the latter being sometimes red, but more 

 commonly nearly black in colour. It has often been supposed 

 that the Cambrians 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 ap- 

 pear to have been laid down in shallow water, as they exhibit 

 rain-prints, sun-cracks, and ripple-marks incontrovertible evi- 

 dence 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 gen- 

 eral absence of limestones, coupled with the coarse mechani- 

 cal nature of many of the sediments 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 towards 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 the 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 

 to be rather referable to the tracks and burrows of marine 

 worms. The first-mentioned of these formations has also 

 yielded the curious, furrowed and striated stems which have 

 been described as a kind of land-plant under the name of 

 Eophyton (fig. 28). It cannot be said, however, that the vege- 

 table origin of these singular bodies has been satisfactorily 

 proved. Lastly, there are found in certain green and purple 



