CHAP. IV.] SILURIAN PERIOD. 45 



Silurian conglomerates were deposited along its shores, 

 while the sand and mud that was worn from the detrition 

 of the land was carried eastward and spread over the 

 bottom of the neighbouring sea. 



Silurian rocks occur also in Kerry, and include represen- 

 tatives of nearly the whole system, but the actual base is 

 not seen, while their upper limit is difficult to fix, as they 

 pass into a series of slates and grits which are litho- 

 logically similar to the beds below, but do not contain 

 fossils. 



2. Geographical Restoration. 



We have seen that nearly everywhere, but especially in 

 the north-west of Ireland, there was great disturbance and 

 upheaval at the commencement of the Silurian period ; 

 that this was followed by a gradual subsidence, during 

 which the materials gained from the land tracts were 

 quietly spread over the sea-bottom ; and that, finally, over 

 the English and Scottish areas, the incoming of sandstones 

 and the narrowing of the areas of deposition bespeak a 

 rapid elevation. 



It will be best to consider the geographical conditions 

 which seem to have existed at the beginning of the period, 

 for we cannot say how much of the land areas remained 

 during the subsequent submergence. 



The largest land tract seems to have been to the north 

 and north-west of the British Isles, and there is reason to 

 think that a continent of some size occupied a large part 

 of what is now the North Atlantic Ocean. No Silurian 

 rocks having been found in the northern parts of Scot- 

 land, it is probable that in early Silurian times the coast- 

 line of this continent ran through the north-west of Ire- 

 land and the centre of Scotland. The Ayrshire conglome- 

 rates were probably formed round an island, or possibly off 



