444 SCIENTIFIC RECORD FOR 1883. 



we are led to look to a modified Neptunian view as a solution of the 

 problem. 



The origin of continents and the supposed stability of sea-basins is 

 ably discussed in the Geological Magazine for June, 1883, by W. O. 

 Crosby, and reasons are there given for calling in question the doctrine 

 lately maintained by Dana and others in opposition to the older and 

 generally received view of the alternations of sea and land. The derived 

 origin of vast continental areas made up of great thicknesses of Paleo- 

 zoic and more recent sediments, in great part of mechanical origin, 

 necessarily involves the destruction and the disintegration by chemical 

 and mechanical processes of not less considerable masses constituting 

 previous continents. 



In this connection Hunt has discussed the question of the sub-aerial 

 decay of rocks. (Amer. Jour. Science, September, 1883.) This process 

 consists in a more or less complete chemical decomposition of most of 

 the silicates of the crystalline rocks, the feldspars being converted into 

 clay by the loss of their protoxide-bases and a large part of their silica, 

 all of which are removed in solution in water. The silicates of protoxide- 

 bases, such as hornblende, are in like manner decomposed, with the so- 

 lution of the lime and magnesia, and most of the silica, the iron remain- 

 ing as peroxide; while certain silicates, such as garnet and tourmaline 

 resist, like quartz and magnetite, the process of decay. These trans- 

 formations being effected under the influence of the atmosphere, the 

 bases are dissolved as carbonates. Thus the decay of crystalline rocks 

 is " a necessary preliminary to glacial action and erosion, which removed 

 previously softened materials." The points insisted upon by the author 

 are thus resumed: 



1. The evidence afforded by recent geological studies in North Amer- 

 ica and elsewhere shows the universality and the antiquity of the sub- 

 aerial decay both of silicated crystalline rocks and of calcareous rocks, 

 and its great extent in pre-Cambrian times. 



2. The fact that the materials resulting from this decay are preserved 

 in situ in regions where they have been protected from denudation by 

 overlying strata alike of Cambrian and more recent periods, or, in the 

 absence of these coverings, by the position of the decayed materials 

 with reference to denuding agents, as in drift! ess regions or in places 

 sheltered from erosion, as in the Appalachian and St. Lawrence valleys. 



3. That this process of decay, though continuous through later geo- 

 logical ages, has, under ordinary conditions, been insignificant in amount 

 since the glacial period, for the reason that the time which has since 

 elapsed is short, and also, perhaps, on account of changed atmospheric 

 conditions in later ages. 



4. That this process of decay has furnished the materials, not only 

 for the clays, sands, and iron oxides, from the beginning of Paleozoic 

 time to the present, but also for many corresponding rocks of Eozoic 

 time. The bases thus separated from crystalline silicated rocks have 

 been the source, directly and indirectly, of all limestones and carbon- 



