THE CAMBRIAN SYSTEM 107 



the condition of the larger fossils in these beds is such that they 

 must have lain for a long time on the sea-floor and have suffered 

 greatly from decay and partial solution before tli.-y w.-re protected 

 by a covering of other calcareous materials.. 



Mr. Peach ** has observed that the shells are generally imperfect 

 and seldom show the external markings ; the Lamellibranchs, 

 though often found with attached valves and in their natural 

 position, are yet often so imperfect that they have clearly suffered 

 much from solution before complete embedment Again the outer 

 walls and the septa of the chambered Cephalopods have in most 

 cases been largely or wholly destroyed, so as to leave only the 

 thick column of the central siphuncular portion of the shell 



Moreover, these limestones contain many nodules of chert 

 arranged in layers like the flints and cherts in Cretaceous deposits, 

 and sometimes in continuous layers or sheets. These cherts 

 contain sponge-spicules, and though no Radiolaria can now be 

 recognised in them it is very probable that the silica was derived 

 partly from Radiolaria and partly from siliceous sponges. 



The occurrence of glauconite grains in certain beds, not only in 

 Scotland but in the Upper Cambrian of Sweden and Russia, is also 

 indicative of clear water, slow deposition, and some distance from 

 land ; for it is probable that the conditions favourable for the 

 production of glauconite on sea-floors have not materially changed 

 from Cambrian time to the present day. Those conditions are 

 found where the water is clear and the bottom currents are fairly 

 strong but carry little sediment, and where the depth of water is 

 from 200 to 500 fathoms ; the actual depth, however, seems to be 

 less important than the other conditions. 



Passing now to the more southern parts of the European region 

 we find proofs of the existence of a Cambrian Sea in Portugal, 

 Spain, Provence, and Sardinia ; so that it seems probable that the 

 whole of Western and Southern Europe was covered by this sea, 

 though there may have been islands within its limits. It is only 

 in Bohemia that we seem to approach one of the borders of this 

 sea, for there the older Cambrian deposits are limited to one part 

 of the area, the Upper Cambrian overlapping on to Archaean rocks. 

 Even this land may have been an island, but if it was part of a 

 large tract that must have lain to the east or south of Bohemia 



Another point to be noted with regard to the physical conditions 

 of Cambrian time is that very little volcanic activity showed itself 

 in the European region till toward the close of the period. After 

 the violent and long-continued outbursts of volcanic energy which 

 prevailed in pre-Cambrian times, there seems to have been a period 

 of quiescence and subsidence before the subterranean forces acquired 



