24 DE. THOMAS STERRY HUNT ON THE 



inclusion in this of what so many regard as an organic form, the Eozoon Caruidense ; the 

 presence, alike in the limestones, gneisses and associated quartzites, of carbon in the form of 

 graphite ; and, finally, the occurrence of sulphids, testifying to a process of reduction of 

 sulphates (which, not less than the graphite, suggests organic matter,) all indicate chemical 

 processes such as are now going on at the earth's surface, and have been in operation since 

 the beginning of paleozoic time ; but which are inconsistent with any considerable eleva- 

 tion of temperature above that now prevailing on the earth. They are, in short, evidences 

 that the processes of vegetable and animal life were going on simultaneously with the deposi- 

 tion of the rocks of the Laurentian period. More than this, the presence of rounded masses of 

 older gneisses in the yoimger crystalline schists, not less than the composition of these schists 

 (as we shall hope to show in the sequel), are evidences that during the period in question 

 a subaërial decay of the older crystalline rocks was already going on, giving rise to boulders 

 of decomposition, to clays, and all the chemical reactions which that process implies, and 

 which I have elsewhere set forth at length.^' 



§ 46. If we have correctly defined the conditions requisite for the production of the 

 crystalline stratified rocks, they must have been separated from water by a i)rocess of 

 crystallization or precipitation, at a temperature and a pressure not widely different from 

 those now prevailing at the earth's surface. This process, in the earlier periods, must have 

 been widely extended, and, so far as known continental areas were concerned, probably 

 universal. A slowly progressive change meanwhile went on in the chemical conditions, 

 indicated by a gradual modification in the composition of the rocks, and the areas of 

 deposition, though still very great, became limited, leaving large surfaces, both of subse- 

 quently erupted rocks and of the precipitated stratified rocks, exj^osed to a process of sub- 

 aerial decay, the soluble and insoluble products of which alike intervened in the rock- 

 forming processes of this later or transition period. The conditions of the problem before 

 us require moreover a source, neither detrital nor volcanic, for the immense mass of wholly 

 crystalline material, chiefly quartz and feldspars, con.stitutiug the vast and as yet 

 unfathomed i^rimitive granitic and gneissic series ; which only at a later time furnished 

 its contingent of decayed and detrital matter to the crystalline transition rocks. 



But there is still another condition imposed by the problem before us — that of a satis- 

 factory explanation of the highly inclined and often nearly vertical attitude of the crystal- 

 line stratified rocks, which is most remarkable in those of the earliest periods. The 

 ordinarily received explanation of this, as due to the contraction of a cooling globe, has 

 seemed so inadequate to account for the great contortion, crushing, and folding of these older 

 rocks, that some geologists, as Naumann tells us, have been led to regard the present 

 as their original attitude, resulting from movements of the solidifying crust ; in which 

 connection he quotes with approval the language of Kittel, that '• so long as a hypothesis 

 is unable thoroughly to explain the almost vertical position of the primitive strata, it 

 cannot be regarded as even approximately near the truth." 



It will, we think be apparent, in the light of the preceding review of existing hypo- 

 theses, that no explanation of the origin of the crystalline rocks which fails to meet all of 

 the conditions just defined can hope for the approval of those who, after a careful survey 

 of the whole field, seek for a new and more satisfactory hypothesis. It remains to be seen 



»^ The Decay of Rocks Gteologically Considered,U883. — Amer. Jour. Soi., vol. xxvi., pp. 190-213. 



