On the Structure and Position of Eozoon Canadense. 279 



granite on which they rested, and consisting of gneiss, mica- 

 schist, clay-slate, quartz, alum-shale, grauwacke, with occa- 

 sional limestone beds. The older of these rocks were not 

 supposed to contain any fossil remains ; and from the indica- 

 tions they presented of changes in their condition subsequent 

 to their original deposition, they were distinguished as meta- 

 morphic. In the newer strata, on the other hand, the presence 

 of fossils had been recognized; but no such comparison had 

 been made between these and the fossils of the Old Red Sand- 

 stone and Carboniferous Limestone (which were the oldest strata 

 that had then been systematically studied), as threw any light 

 on their mutual relations. 



It was a little more than thirty years ago that Sir Roderick 

 (then Mr.) Murchison was enabled, by the careful study of the 

 order of superposition of the newer of these rocks in South 

 Wales and along the Welsh border, to establish the existence 

 of a regular series of strata, graduating downwards continuously 

 from the lower beds of the Old Red Sandstone, and charac- 

 terized by a distinct and peculiar assemblage of organic 

 remains ; and this series he designated the Silurian system, 

 the region which had first revealed its existence being that 

 once inhabited by the ancient Silures. The base of this series 

 (which was marked out into Upper and Lower Silurian, both by 

 a want of geological conformity and by differences in the 

 fauna of the two divisions) was supposed to be formed by the 

 hard beds of fissile sandstone largely developed near the town 

 of Llandeilo in Carmarthenshire., and known as the " Llandeilo 

 flags." These rest unconformably upon what was then 

 designated the " Clay-slate" system ; and it seems to have 

 been at first assumed by the investigator of the Silurian system, 

 that organic life had no existence on the globe previously to 

 the epoch thus marked out. 



But whilst Sir Roderick Murchison was thus working out 

 in South Wales the later portion of the series of " primary 

 rocks," Professor Sedgwick was applying himself with equal 

 zeal and energy to the study of the older, as displayed in 

 Cumberland and North Wales ; and he succeeded in proving 

 that whilst a regular order of superposition, broken however 

 by many disturbances, may be traced through that massive 

 series of slaty rocks of which a large proportion of the moun- 

 tain ranges of Cumberland and North Wales are composed, 

 the newer of these rocks present fossil remains in sufficient 

 numbers to show that it was incorrect to assume the absence 

 of life in those ancient seas. Thus a thick stratum of fissile 

 sandstone underlying the " Tremadoc slates," which, again, 

 lie beneath the Llandeilo flags, contains such a vast abundance 

 of shells of the still existing genus Lingula, that this stratum 



