158 Kenry Hicks — On the Nor them Palceozoic Rocks. 



the Cambrian, marine life helped in making up the formations in nearly- 

 all the areas, and in some cases, as may be seen on referring to the 

 Folding Table, to build them up almost entirely. By the close of 

 the Lower Silurian, the pre-Cambrian crust had been depressed along 

 the south-western borders of Europe to a depth of over 30,000 feet, 

 and sediments to that extent had in some cases been heaped up. 

 Along the eastern borders of North America sediments had also 

 accumulated to a depth of over 18,000 feet,^ so the depression there 

 must also have been great. Over other areas the sediments were 

 comparatively thin, and the crust at those parts but slightly sub- 

 merged. In many of these places the faunas also are such as would 

 indicate only a moderately shallow sea. In some parts in Asia, in 

 America, and also in Europe, the higher points of the pre-Cam- 

 brian ridges were still above the water-level. Hence it is evident 

 that the series up to this time must have varied greatly in thickness, 

 and generally in proportion to the unevenness of the pre-Cambrian 

 crust. In Lower Silurian time, volcanic action took place over 

 limited areas, and the sediments at those parts were greatly added 

 to by the ejectamenta, and therefore heaped up rapidly. In Great 

 Britain, and the south-west of Europe, and also in the neighbour- 

 hood of Lake Superior^ in North America, the Lower Silurian 

 rocks are made up in part from volcanic materials, as tuff or 

 contemporaneous trap, and ashes re-arranged by water. The 

 volcanos, which at first appear to have been entirely submarine, 

 had heaped up, before the close of the Lower Silurian, by repeated 

 eruptions in the British areas of North Wales and Cumberland, 

 sufficient material to reach in some places considerably out of the 

 water, and hence to form islands. During the later period of these 

 eruptions, certain movements terminating in the elevation of portions 

 of the earth's crust took place also over limited areas in Western 

 Euro^De and in North America. In Western Europe these move- 

 ments seem to have been confined to the immediate regions of 

 volcanic action, but in North America they were apparently unac- 

 companied by any direct outbursts, though the uplifted portions show 

 indications that the sediments had been subjected to the influence of 

 great heat f greater in my opinion than what could be produced by con- 

 tortion and upheaval only, if the evidence in other rocks which have 

 undergone contortions only can be relied on. This break, the first 

 in the Palseozoic rocks, is entirely confined to portions of the areas 

 first submerged and usually to the deeper parts, and where the 

 greatest thickness of sediment had accumulated on the Pre-Cambrian 

 crust. Immediately after these movements had taken place, dejores- 

 sion seems to have gone on again over those areas, and parts v/hich 

 had been raised above the water were once more covered by the sea, 

 and sediments thrown down upon them. It will be seen, therefore, 

 that these breaks are not signs that there was an arrest in the 

 subsidence generally over even these areas. Over the larger areas, 

 it is certain that the contraction of the crust at this period continued 



1 H. D. EogerSjDanajetc. 2 Foster, "Whitney, Owen, etc. 



3 Green Mountain Eegion, Logan, according to Dana, Hunt, etc. 



