AcniTST .11, 1(!83.] 



SCIENCE. 



293 



called l>y Renloaiix ' K.ip*oI riiiliTw-erkp,' or 'cliam- 

 ber-i-raiik irains." Moconliiii; to Ki'iinody. 



List of other papers. 



TliP fallowing additional papers wore read in lliis 



section, some of tliein by title only: — A inetliod of 



testing long plane surfaces, applicable to llie align- 

 ment (>f,pl;nier-bed3. latlie-bcds, heavy sliafling, etc., 

 by ir. A. litiyeri. The commercial and dynamic 

 efficiencies of the steam-engine ; Centrifugal action 

 ill turbines, by li. II. Tlmrxlon. Velocity of the 

 piston of a crank engine, by V. M. Woodioard. 



PROCEEDINGS OF SECTION E. — GEOLOGY AND GEOGRAPHY. 



Anniu-:ss of c. ii iiitchcock or iian- 



(>yi:/!, A'.//.. VlCli-PliHSIDEiSr UF 6EC- 

 'JIO.X E, A U(J. 15, 1SH3 



THE EARLY HISTORY OF THE NORTfl- 

 AMEKlfAN CONTINENr. 



TiiKiiK is a special appropriateness in the associa- 

 tion of geography willi geoloyy, as indicated in the 

 assignment of j-ciences to .section E; for the latter 

 gives us an account of the origin of every topograph- 

 ical fiatiire of the earth's surface, whether i.sland, 

 coiitiiient, mountain, plateau, valley, or oceanic de- 

 l^rcssicn. If we would properly umlerstand the sig- 

 iiiHcance of the earth's contours, we must unravel 

 the mysteries of geology: .-.o a knowledge of topog- 

 raphy is essential to the complete comprehension of 

 tlie geological fi'atures of any country. Jf a geologist 

 Were taken by a balloon to an unexplored part of the 

 earth, lie would instantly recognize, from their topo- 

 graphical outlines, volcanic and granitic cones, lime- 

 stone hills, elevated plateaus of basalt or horizontal 

 sinilsiimes, and special types of orographic structure. 

 Jlence the moilern geologist first draws the contours 

 of bis district before applj ing the colors of geological 

 age. The existing relief features of the earth have 

 been produced one by one in successive periods; and 

 it i» the task of the geologist to discover what were 

 the characteristic ]>liysical developments of the 

 >everal ages. He can deliin-ate a connected historical 

 >keteh of the beginning, growth, and completion of 

 a continent. Such histories are rare, because atten- 

 tion has but recently been turned into this direction. 

 One of the first American geologists to fiame such 

 an imtline is Prof. J. f). Dana, to wlumi we owe the 

 enunciation of ihU fundamental truth. — that the 

 iirst foinied land has always remained above water, 

 and ba-s been a nucleu.' about which zones of sedi- 

 iu( lit have acciminlated. We can now recognize 

 this primitive continent, with all its successive stages 

 of growth, upon every geological map. 



Time would fail us to present the entire physical 

 liislory of our continent; and we will therefore confine 

 our attention chiefly to its earlier chapters. n<iting 

 those points which aie under di.-cu»ion. A^ we are 

 endeavoring to iidcnncv !.eience, we must touch upon 

 debatable to|iics, and hope by friendly discussions 

 to become wUer. 



AVe mu«i assume the correctness of the comninnly 

 received oiiinioiis concerning tlie earliist history of 

 our planet. — that it passe<l throngh the condilion 

 of a nebuKa, and then of a burning sun, the period of 



igneous fluidity. By subsequent refrigeration it has 

 liecoine either partially or wholly solid. Not until a 

 crust had formed, and the earth liad cooled enough 

 to allow water to remain permanently, was it pos- 

 sible to talk of dry land and oce^ii. With these prem- 

 ises allowed, it seems to us evident that the material 

 of the earth must be disposed in concentric zones, 

 arranged according to density, the heaviest being at 

 the centre. If the various elements were free to 

 move, as is the case in all natural or artificial igneous 

 fluids, we must expect to find the heavier metals 

 situated beneath the others; and, following the 

 analogy of extra-terrestrial bodies, the central nucleus 

 may be principally iron, like the heavier meteors. 

 Zones corresponding to stony meteors, lavas, the trap 

 family, and granites would naturally succeed in order, 

 the last named being at the surface. This outer 

 zone is also characterized by the presence of much 

 silica and oxygen. The primeval ocean came from 

 the vapors surrounding the igneous sphere, con- 

 densed to liquidity as soon as water could remain 

 upon the solid crust without immediate v;ipo.rizatii>ii. 

 This original crust may have been essentially a. 

 plain, and consequently entirely covered by water; 

 for if the land were now levelled off, the ocean woiihl 

 submerge every acre of the continents. As refrige- 

 ration progressed, ridges and valleys W(nild form 

 in accordance with that fundamental principle that, 

 the outer envelope must conform to the shrunken 

 nucleus; and this contraction gives rise to that 

 tangential force or lateral iiressure which has acted 

 through all time. Whether these earliest ridges lose 

 above the ocean would depend upon the aniuiint of 

 elevation. Some authors argue that these ridgi^s fol- 

 low the course of great circles. If there are causes 

 adequate to produce such results — or any other 

 world-wide arrangement, — they must have roin- 

 menced to operate at the very beginning of contrac- 

 tion. Most authors maintain that the very thick 

 strata of the older rocks have been formed just like 

 modern sediments, having been broken off the ledges, 

 and transported into oreanic basins in horizontal 

 attitude. If so, there must have been great moun- 

 tainous elevations, deep cceanic depression.!, and 

 extensive aqueous action ; since the thickness of the 

 crystalline .schist< is greater than that of. the strata 

 in the fossiliferoiis ages. The amount of distortion, 

 crumpling, and faulting of the crystalline rocks is 

 also greater. These .same authors hold that the 

 original strata were in all respects like inodern sands, 

 gravels, and clays, and that their present cryxtallinu 

 structure is due to metanioi|diism. No one has yet 

 discovered any unci^slalliue pre-C'ambiian beds; nor 



