258 STRUCTURE COMMON TO ALL ROCKS. 



Mountain-Ranges were once Marginal Sea-Bottoms.— Where, then, 

 do sediments now accumulate in greatest thickness? Evidently on 

 marginal sea-bottoms, off the coasts of continents. The greater part 

 of the washings of continents are deposited within 30 miles of shore, 

 and the whole usually within 100 miles. From this line of thickest 

 and coarsest deposit the sediments grow thinner and finer as we go sea- 

 ward. But evidently such enormous thicknesses as 40,000 feet can not 

 accumulate in the same place 'without pari passu subsidence such as we 

 know takes place now whenever exceptionally abundant sedimentation 

 is going on (p. 139). Therefore, mountain-ranges before they were yet 

 bom — while still in preparation as embryos in the womb of the ocean 

 — were lines of thick off-shore deposits gradually subsiding , and thus 

 ever renewing the conditions of continuous deposit. 



As this is a very important point, it is necessary to stop here awhile 

 in order to show that such was actually the fact in the case of all the 

 principal ranges of the American Continent — i. e., that for a long time 

 before they were actually formed, the places which they now occupy 

 were marginal sea-bottoms receiving abundant sediments from an ad- 

 jacent continent. We shall be compelled here to anticipate some things 

 that belong to Part III, but we hope to make statements so general 

 that there will be no difficulty in understanding them. 



1. Appalachian. — The history of this range is briefly as follows : 

 At the beginning of the Palaeozoic era there was a great V-shaped 

 land-mass, occupying the region now covered by Labrador and Canada, 

 then turning northwestward from Lake Superior and extending perhaps 

 to polar regions about the mouth of Mackenzie River. This is shown 

 on map, Fig. 266, on page 292. There was another great land-mass oc- 

 cupying the present place of the eastern slope of the Blue Ridge and ex- 

 tending eastward probably far beyond the present limits of the conti- 

 nent — as shown in the same figure by dotted line in the Atlantic Ocean. 

 The western coast-line of this land-mass was the present place of the 

 Blue Ridge. Westward of this line extended a great ocean — " the in- 

 terior Palaeozoic Sea." The Appalachian range west of the Blue Ridge 

 was then the marginal bottom of that sea. During the whole of the 

 Cambrian, Silurian, and Devonian, this shore-line remained nearly in 

 the same place, although there was probably a slow transference west- 

 ward. Meanwhile, throughout this immense period of time, the wash- 

 ings from the land-mass eastward, accumulated along the shore-line, until 

 30,000 feet of thickness was attained. At the end of the Devonian some 

 considerable changes of physical geography of this region took place, 

 which Ave will explain when we come to treat of the history of this pe- 

 riod. Suffice it to say now that during the Carboniferous the region of 

 the Appalachian was sometimes above the sea as a coal-swamp, and some- 

 times below, but all the time receiving sediment until 9,000 or 10,000 



