VII.] ON SOME POINTS IN DYNAMICAL GEOLOGY. 75 



and for the thinner and undisturbed strata of the valley of 

 the upper Mississippi. The hills in the latter region, built 

 up of 1,000 feet of horizontal beds, having the Potsdam 

 sandstone at the base, and capped by the Niagara limestone, 

 show us the production of mountains by erosion, uncompli- 

 cated by the accidents which make their study more difficult 

 in regions where contortion of the strata has supervened. Hall 

 has also noted in this connection the nearly horizontal strata 

 of the Catskill Mountains. 



The question of the structure and the origin of the Appa- 

 lachians has been complicated by the assumption that the 

 crystalline strata which constitute their higher portions are 

 altered sediments of palaeozoic age, rather than parts of an an- 

 cient continent of eozoic rocks which formed the eastern bor- 

 der of the palaeozoic sea, corresponding to the Eocky Moun- 

 tains on the west. The former view has been very generally 

 held by American geologists, and was maintained by the pres- 

 ent writer until 1870, when he endeavored to shoAv that the 

 crystalline rocks of New England, and their lithological repre- 

 sentatives , both to the southwest and the northwest, are of 

 pre-palseozoic age, and in part Laurentian. (Amer. Jour. Sci- 

 ence (2), L. 83; also Address before the American Association, 

 Indianapolis, 1871, Paper XIII. of this volume.) This view, 

 already before maintained by Credner and by Emmons, is now 

 advocated by LeConte, who conceives that the gneissic region 

 of the Atlantic slope is Laurentian, and was probably " land 

 during the paleozoic times," constituting an " eastern . conti- 

 nental mass," which, judging from the immense thickness of 

 sediments in the eastern parts of the palaeozoic area, must have 

 been of great extent. A similar view was put forward by 

 H. D. Eogers in 1842, and again by James Hall in 1859, 

 when, after describing these palaeozoic sediments, he said, " We 

 may have had a coast-line nearly parallel to and coextensive 

 with the Appalachians " (Paleontology, Vol. III. p. 96, note) ; 

 commenting upon which, in 1861, I asserted that these coarse 

 sediments " were evidently derived from a wasting continent." 

 In a paper read before the American Geographical Society, 



