GEOLOGY OF NORTH ATLANTIC — GILLIGAN 215 



IV. THE CAMBRIAN, ORDOVICIAN, AND SILURIAN PERIODS 



I have grouped these together because, taken on the whole, the 

 sedimentary deposits are very similar. Locally they show great 

 variation and ever-changing physical conditions. In western and 

 northwestern Europe we get evidence of the proximity of shore lines 

 with clear and possibly deeper seas extending to the east. If we 

 consider the deposits on a line from North Wales to the Baltic 

 Provinces of Russia and take the thicknesses of these protozoic rocks 

 at different points along the line it will be found that they dwindle 

 from some 30,000 feet in Wales to a few hundreds in the Baltic region, 

 and that while they are very largely of clastic materials in the west 

 they are organic limestones and graptolitic shales in the east. 



In the Cambrian rocks of Scotland we have definite evidence of a 

 shore line separating a land area to the northwest and a sloping sea 

 floor to the southeast, and the oscillations of the shore line and there- 

 fore of the uplift and depression of the land and sea floor, during the 

 accumulation of the Cambrian deposits of that area. 



Similar conditions in Ordovician and Silurian times can be traced 

 in the deposits accumulating in the neighborhood of Girvan, and the 

 Southern Uplands doubtless was the site of coterminous deltas drain- 

 ing a northern land. I need only mention two of the beds which are 

 to be regarded as derived at first hand from rocks of granitic type 

 lying probably to the west of the present deposits ; namely, the Har- 

 lech grits of the Cambrian and the massive Denbighshire grits of 

 the Silurian. 



The Lower Paleozoic deposits of North America form the mirror 

 image of those on the European side in that to the east are the 

 coarser sediments followed by shales farther westward, and these 

 again still farther to the Vvcst by limestones, all these deposits being 

 contemporaneous. There is also traceable a continual overlapping 

 of the newer beds upon the older from Avest and southwest to east 

 and northeast, e. g., at Cape Breton the Lower Cambrian is several 

 thousand feet thick, while 30 miles to the northeast at St. John, New 

 Brunswick, these are only 1,200 feet, and the Middle Cambrian com- 

 pletely overlap the Lower Cambrian still farther to the northeast. 

 This clearly points to a depression of the land areas to the east and 

 northeast and an encroachment of the waters which lay to the west 

 and southwest. 



V. THE OLD RED SANDSTONE PERIOD 



The Old Red Sandstone, like the Torridonian, may be described 

 as a continental deposit. The distribution of the marine Devonian 

 and the continental Old Red Sandstone in the British Isles and Eu- 



