294 



SCIENCE. 



[Vor,. II., No. .-Ml. 



have the original foundation rocks been pointed out, 

 since the oldest known layers are stratified, and 

 cannot therefore have constituted part of the original 

 unstratified crust. 



Professor Dana thinks the primitive land originated 

 because of a difference in the rate of conduction 

 of heat during the process of refrigeration. Cooling 

 ■n'ould be fastest where the heat was conducted most 

 rapidly. The first areas to cool would be the first to 

 solidify. The first solidified crust was heavier than 

 the adjacent liquid: so It sank until it found a 

 fluid as dense as itself. Then the liquid above this 

 crust would in turn become solid, and sink; and this 

 process is supposed to have continued until a perma- 

 nent shell had become fixed in the earth's circum- 

 ference, which constantly increased in breadth and 

 thickness, becoming continents. Meanwhile the 

 other portions remained liquid; and iheir surfaces 

 must have stood at the same level with the first- 

 formed crust till that congealed, andbecamedepressed 

 because of the diminution of volume in solidifica- 

 tion. These depressions became the ocean's beds. 

 From this beginning down to the present time the 

 processes of growth have consisted in the thicken- 

 ing of the continents and the settling-down of the 

 oceanic depressions, while the chief force employed 

 has'been the lateral pressure derived from contrac- 

 tion. LeConte and Pratt express the process thus far 

 described by the term 'unequal radial contraction.' 

 The total gravity of the particles of matter along 

 each radius is supposed to be the same; and hence, 

 if the heat is conducted most rapidly over the shorter 

 radii composed of denser minerals, the ocean-basins 

 would cool first. These two views thus demand a 

 different arrangement of the lighter and denser 

 materials; the one necessitating that the continents, 

 and the other the depressions, were first to congeal. 

 Both, however, make the gratuitous and unproved 

 assumption, that the surface was not uniform in 

 composition; the differences being probably like 

 that between granite and trap. The principle stated 

 above — that, where all the particles are free to move 

 in a liquid, the lighter elements must rise to the sur- 

 face, and the heavier minerals sink down in propor- 

 tion to their specific gravity —is at variance with this 

 assumption. Fortunately it is not essential to a right 

 theory of continental growth. There is no reason, 

 therefore, to doubt that the original cmst had 

 essentially a uniform thickness over the whole 

 earth. Contraction would originate ridges and val- 

 leys in the normal way, most likely of similar 

 dimensions. There must soon burst forth ejections 

 of igneous matter, owing to tidal attraction; and 

 these would show themselves along the weakest lines. 

 At the outset it is difficult to assign reasons why 

 either the elevations or depressions would be the 

 weaker; and hence we should look for a multitude 

 of locations of igneous overflow, both over the future 

 continents and oceans. There may be no better 

 reason for the eventual enlargement of certain of 

 these volcanoes than that circumstances only very 

 slightly favored them; but, this favor being con- 

 tinued, they would exist and enlarge at the expense 



of the others, affording us another illustration of the 

 ' survival of the fittest.' 



It seems to us there is now afforded an opportunity 

 for reviving in a modified form the view of Poulett 

 Scrope ill regard to the origination of the earlier 

 crystalline deposits. Suppose we say, that, besides 

 the original unstratified igneous granitic material, 

 the oldest stratified crystalline rocks are derived 

 from volcanic ejections ; being the continued enlarge- 

 ment in size, and reduction in number, of the early 

 indeterminate vents. The several ejections would 

 increase in size till they became islands, either gneis- 

 sic or granitic; and, if an archipelago is allowed us, 

 we can easily show how continents would accumulate, 

 using only the universally acting forces of lateral 

 pressure and sedimentary accumulation. 



Of other theories relating to the origin of the 

 earlier crystalline beds I may mention two. The 

 first is that advocated by Lyell, who termed these 

 rocks hypoijene. After the solid granitic crust had 

 been formed by refrigeration, " the hot waters of the 

 ocean held in solution the ingredients of gneiss, 

 mica-schist, hornblende-schist, clay slate, and marble. 



— rocks which were precipitated one after the other 

 in a crystalline form" (Lyell's Principles of geology, 

 10th ed., i. 142). In such a menstruum, life could 

 not have existed. A very similiar view was advocated 

 by Dr. T. Sterry Hunt in his presidential address 

 before this association in 1S71. 



The second is the view more commonly entertained, 



— that, after the solidification of the crust, sedimen- 

 tation accumulated sti'atified systems from the 

 granitic foundations, as ordinary sand, gravel, and 

 clay, which were subsequently acted upon by thermal 

 and aqueous influences termed metamorphic, and 

 thus converted into crystalline schists. The wide- 

 spread and powerful action of metamorphism is 

 conceded ; but it is a more appropriate adjunct to 

 volcanic than sedimentary accumulation. 



A few of the considerations favoring our theory 

 will now be presented. 



1. Considering the igneous origin of the earth, 

 volcanic energies would naturally continue their 

 action as soon as there was a crust to be broken 

 through, and immense piles of melted rock would 

 ooze from the numerous fissures. Up to Laurentian 

 times all admit the universality of igneous outflow, 

 while but few have ventured to speak of any thing 

 like volcanic action, except as it has been manifested 

 in the formation of dikes in these early periods. 

 There has been a tendency to class the ancient 

 granites and porphyries with rocks of sedimentary 

 origin, and consequently to restrict the action of 

 igneous agencies to phenomena of slight importance. 

 Several English writers, and, in our country, Dr. 

 Selwyn of Canada, have been calling our attention 

 to the existence of a volcanic group in later Huronian 

 or early Cambrian times. These are the rocks so 

 largely developed about Lake Superior, New Eng- 

 land, and the Province of Quebec, consisting of 

 stratified schists, diorites, diabases, amygdaloids, and 

 felsites, identical in composition with true eruptive 

 masses of the same name. Investigation shows that 



