14 THE ORIGIN AND ALTERATION OF ROCKS. 



metamorphic product, i. e., not derived directly from the eruptive rocks. 

 These minerals are characteristic not only of the modern lavas, but also 

 of the most ancient eruptive rocks in which secondary alteration has not 

 obliterated their characters; not only in the moderu basalt, but also in the 

 ancient melaphyr; not only in the modern rhyolitc, but also in the ancient 

 felsite. These characters are not confined to any single locality or age, but 

 are, so far as known, world wide, and go back to the earliest times in which 

 such rocks occur.* 



We should then claim that the field evidence, as well as the microscopic, 

 is opposed in Mo to the prevailing theory that the eruptive rocks are derived 

 from sedimentary ones. That theory demands immense duration of time, 

 unstable continents, enormous foi'ces, a solid earth that shall be more rigid 

 than glass, and yet yield like a rubber ball to the slightest pressure of sedi- 

 ments, lava flows, or glaciers. The theory in question demands the removal 

 of immense masses from one place, and their deposition in another, the ele- 

 vation of billions of tons in order to avoid the necessity of admitting the 

 elevation of hundreds, — for in order to have denudation that shall brintj 

 once deeply buried sediments to the surface, the entire mass must be lifted 

 bodily above the surrounding region, or from the zone of aqueo-igneous 

 fusion to the outer air. Which view requires the greatest force — to elevate 

 and depress such enormous bulks in a solid earth, or to raise our lavas from a 

 liquid interior — is plainly evident. This theory requires that volcanic action 

 should be of modern birth — Tertiary — and that eruptive rocks of earlier 

 date should have been produced by diiferent forces — a view now known to 

 be false. To the theory that the crystalline rocks are chemical precipitates 

 arranged in regular succession, there arises the serious objection that the 

 oldest form — the so-called Laurentian — is cut by dikes of rocks whicli, 

 according to that theory, could not have existed below them ; that is, they 

 belong lithologically to the so-called Huronian and Norian systems. 



In contradistinction to the views here indicated, the writer's petrographical 

 studies lead him to hold, with some others, that all volcanic or eruptive 

 action arises from the original igneous state of the earth — that it must have 

 begun in the earliest ages of this globe. This action being a manifestation 

 of a dying energy, must have been more active in the past than at present, 

 although it may have been intermittent in character as all such forces seem 

 to be. The products of this action have been the same from the earliest to 



-* See also David Forbes. Nature, 1870, ii. 283-286 ; 1873, vii. 259-361. 



