58 DR. THOMAS STEEEY HUNT ON THE 



studies of various sandstoues.^^'" This fact has been coniirmed by the observations of Young, 

 Irving, and Wadsworth in the United States ; '"^ and Bonney has suggested the possible 

 extension of such a process to feldspar, hornblende and other minerals."" 



Vanhise has very recently announced that his microscopical examinations of cer- 

 tain sandstones of the Kevpeenian series, from Lake Superior, afford evidence of the second- 

 ary deposition of both orthoclase and plagioclase feldspar, in ciystallographic continuity 

 upon broken feldspathic grains, in one case uniting the two parts of a broken feldspar- 

 crystal. The sandstones which have yielded these examples are made up in part of feld- 

 spathic fragments, and in part of fragments of "some altered basic rocks." They are more- 

 over, interstratified with and, in some case at least, immediately irnderlie the basic plutonic 

 rocks of the same Keweeniau series.'"^ When we consider that orthoclase is a common 

 secretion of these basic rocks, as is shown by its frequent occurrence in them with zeolites 

 and epidote, it may perhaps be questioned whether the secondary feldspar in the sand- 

 stone has been derived from the adjacent grains of this mineral, or has come into solution 

 from the transformation of the basic rocks. The apparent stability and insolubility of 

 orthoclase and oligoclase at high temperatures in the presence of water, as observed by 

 Daubrée, would seem to favour the latter view. In any case, it is a striking illustration 

 of the tendency of mineral species to crystallize around nuclei of their own kind, which is 

 so marked a factor in the development of the crystalline rocks, 



IV. — Conclusions. 



§ llï. We reviewed in the first part of this essay the history of the different hypotheses 

 hitherto proposed to explain the origin of the crystalline rocks and, in doing so, reached the 

 conclusion that not one of them affords an adequate solution of the various problems i)re- 

 sented by the chemical, mineralogical and geognostical characters of the rocks in question : 

 at the same time, we endeavored to show succinctly what are the principal conditions to 

 which a satisfactory hypothesis must conform. In the second part, we sketched the growth 

 and development, during the last quarter of a century, of what we believe to be such a 

 hypothesis. In the third part, we sought to bring together a great number of facts, both 

 new and old, which serve to illustrate the new hypothesis ; according to which the crystal- 

 line stratiform rocks, as well as many erupted rocks, are siipposed to have been derived 

 by the action of waters from a primary superficial layer, regarded as the last portion of the 

 globe solidified in cooling from a state of igneous fluidity. This, which we have described 

 as a basic, quartzless rock, is conceived to have been fissured and rendered porous during 

 crystallization and refrigeration, and thus rendered permeable to considerable depths to the 

 waters subsequently precipitated upon it. Its surface being cooled by radiation, while its 

 base reposed upon a heated solid interior, upward and downward currents would establish 

 a system of aqueous circulation in the mass, to which its porous but unstratified condition 

 would be very favorable. The materials which heated subterraneous waters would bring 



'"= Sorby, Presidential Address, Quar. Jour. Geo. Soc. London, xxxvi., 33. 



'"■' Young, Amer. Jour. Sci., xxiv., 47. Irving, Ibid, sxv., 401. AVadsworth, Proc. Boston Soc. Natural History, 

 Feb. 7, 1883. 



"" Bonney, Quar. Jour. Geol. Soc. xxxix., 19. 

 "^ Vanhise, Amer. Jour. Sci., 1884., xxvii., 399. 



