Page:Popular Science Monthly Volume 15.djvu/321

 the Wahsatch and Humboldt Mountains, from 12,000 to 14,000 feet, in the Park and Medicine Bow Ranges somewhat less, and in the Clear Creek region of Colorado at least 25,000 feet. This upper group will be recognized by geologists as closely resembling the Huronian rocks of the East. The Archæan nucleus of the Black Hills was reported by the late Mr. Henry Newton to be composed of two groups of crystalline rocks closely resembling those described by Mr. King, and Mr. George M. Dawson found a similar double series in Manitoba and British Columbia. Without absolute proof—which it would be difficult if not impossible to obtain—the inference is at least allowable that the rocks underlying the Palæozoic series in the far West correspond to the Laurentian and Huronian Groups of the Canadian geologists, and therefore that the foundation of the western half of the continent is essentially the same with that of the eastern; and also that there, as here, a broad continental surface of these older rocks supplied by erosion the mechanical material that entered into the composition of the Palæozoic sediments, which, by successive oscillations of sea-level, were spread to varying altitudes upon its flanks.

At the close of his chapter on the Archæan, Mr. King proposes a theory of the genesis of granite and crystalline schists, which is in some respects new. In common with most of the geologists of the present day, he supposes that the granites and schists are sedimentary rocks which, having locally accumulated to great thickness, have sunk by their own weight into the yielding crust of the earth to a point where they have suffered more or less aqueo-igneous softening, and then, in his view, under varying intensities of radial and tangential pressure, they have been converted into corrugated schists or massive granite, according to the less or greater energy of the forces acting upon them. The evidence adduced by Mr. King to support this mechanical theory of the origin of granite is chiefly derived from the facts which indicate internal and bodily movement in granite, such as the dislocation of inclosed minerals, and the inclusion of masses of foreign rocks.

That there has sometimes been more movement in granites than in the schists with which they are associated—and of which they can frequently be shown to be the exact equivalents in a more metamorphosed condition—is quite certain; but it is very difficult to separate here the effects of force from those of heat. Either produces the practical plasticity which we see recorded in the obliteration of bedding, and the inclusion of foreign rocks. Granites, which exhibit the extreme phase of metamorphism, have evidently been in a plastic state, for they have been forced into fissures of other rocks to form veins and extruded mountain-crests—proofs of softening and movement which schists never afford—but whether this plasticity was the effect of greater heat or greater force than the associated schists suffered, is a question not answered by any facts yet cited. The dislocation of included