igoS.j THE PHYSICS OF THE EARTH. 243 



satch are 30,000 feet thick, when traced eastward thin out to 2,000 feet in 

 the region of the plains. It is evident, therefore, that mountain-ranges are 

 lines of exceptionally thick strata. 



"Mountain-ranges zvere 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 w'ithin 30 miles of shore, and the w-hole usually 

 within ICO miles. From this line of thickest and coarsest deposit the sedi- 

 ments grow thinner and finer as we go seaward. But evidently such enor- 

 mous thicknesses as 40,000 feet cannot accumulate in the same place with- 

 out pari passu subsidence such as we know takes place now whenever 

 exceptionally abundant sedimentation is going on (p. 145). Therefore, 

 mountain-ranges before they n-cre yet born — while still in preparation as 

 embryos in the womb of the ocean — zvere lines of thick off-shore deposits 

 gradually subsiding, and thus ever renewing the conditions of continuous 

 deposits. 



" 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—/, 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 adjacent continent. We 

 shall be compelled to anticipate some things that belong to Part III, but we 

 hope to make statements so general that there will be no difficulty in under- 

 standing them. 



" I. 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 the [Mackenzie River. This is shown on map, Fig. 269, 

 on page 303. There is another great land-mass occupying the present place 

 of the eastern slope of the Blue Ridge and extending eastward probably 

 far beyond the present limits of the continent — 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 interior 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 shoreline 

 remained nearly in the same place, although there was probably a slow 

 transference westward. Meanwhile, throughout this immense period of 

 time, the washings 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 we will explain when we come to treat of the history 

 of this period. Suffice it to say now that during the Carboniferous the 

 region of the Appalachian was sometimes above the sea as a coal-swamp, 

 and sometimes below, but all the time receiving sediment until 9,000 or 

 10.000 feet more of thickness was added, and the aggregate thickness became 



