206 CHEMISTRY OF THE EARTH. 



of tbe scdimentaiy iiiateiial a condition of igneo-aqueons fusion. It 

 would be foreign to our ]>lan to discuss in this place the agencies which, 

 from early geologic periods, have been effecting the transfer of sedi- 

 ments, alternately AA^asting and building-up continents. One, however, 

 requires notice in this connection, namely, the contraction of sediments 

 consequent upon chemical changes, as already explained in § 34, which 

 mast result in subsidence. Such au effect may also result from the ex- 

 travasation of great volumes of liquefied rock, and in either case the 

 depressed portion of the surface becomes a basin, in which sediments 

 may subsequently accumulate, and by their weight upon the yielding 

 stratum beneath continue the process of vsubsidence. While the lower and 

 more fusible strata becomes softened, the great mass of the more silicious 

 rocks, losing their porosity, become cemented into a comparatively rigid 

 mass, and finally, as a result of the earth's contraction, or to counter- 

 balance the depression of some other region, are uplifted as a hardened 

 and corrugateil continental mass, from whose irregular erosion results 

 a mountainous region.* 



§ 53. Those strata which, from their composition, yield, under the 

 conditions just described, the most liquid products, are, it is conceived, 

 the source of all plutonic and volcanic rocks. Accompanied by water, 

 and by difficultly coercible gases, they are either forced among the fis- 

 sures Avhich form in the overlying strata, or find their way to the sur- 

 face. The variations in the composition of lavas and their accomjianying 

 gases in different regions, and even from the same vent at different 

 times, are strong confirmations of the truth of this view. As explained 

 in § 39, the semi-liquid layer of water-impregnated material constitutes 

 a plastic bed, upon which the stratified sediments repose. These, by their 

 irregular distribution over different portions of the earth, determined, 

 after a lapse of time, in the regions of their greatest accumulation, vol- 

 canic and plutonic phenomena. It now remains to show the observed 

 relation of these phenomena, both in the earlier and later times, to great 

 accumulations of sediment. 



§ 54. If we look at the North American continent, we find along its 

 northeastern portion evidences of great subsidence and an accumulation 

 of not less than 40,000 feet of sediment along the line of the Appalachi- 

 ans from the Gulf of St. Lawrence southward, during the paleozoic 

 period, and chiefly, it would appear, during its earlier and later portions. 

 This region is precisely that characterized by considerable eruptions of 

 plutonic rocks during this period, and for some time after its close. To 

 the westward of the Apjialachians, the deposits of paleozoic sediments 

 were much thinner, and in the Mississippi valley are probably less 

 than 4,000 feet in thickness. Conformably with this, there are no traces 

 of plutonic or volcanic outbursts from the northeast region just men- 

 tioned throughout this vast paleozoic basin, with the exception of the 

 region of Lake Supeiior, where we find the early portion of the paleozoic 

 age marked by a gieat accumulation of sediments, comparable to that 

 occurring at the same time in the region of New England, and followed 

 or accompanied by similar plutonic phenomena. Across the ])lains of 

 northern Eussia and Scandinavia, as in the Mississippi valley, the 

 paleozoic period was represented by not more than 2,000 feet of sedi- 

 ments, which still lie undisturbed, while in the British Islands 50,000 

 feet of paleozoic strata, contorted and accompanied by igneous rocks, 



* For a discussion of Ibis subject and tbe tbeory of mountains, inchiding tbe views 

 of Professor James Hall, see tbe autbor on American Geology, American Journal of 

 Science, [2] xxi, 406. 



