Origin of Species Variorum

Comparison with 1861

deposits. In the case of an extensive and shallow sea, such as that within a large part of the Malay Archipelago, where the depth varies from 30 or 40 to 60 fathoms, a widely extended formation might be formed during a period of elevation and yet not suffer excessively from denudation during its slow upheaval; but the thickness of the formation could not be great, for owing to the elevatory movement it would be less than the depth, supposed to be shallow; the deposit would not generally be much consolidated, nor would it be capped by overlying formations, so that it would run a good chance of being worn away during subsequent oscillations of level. It has been suggested by Mr. Hopkins, that if one part of the area, after rising and before being denuded, subsided, the deposit formed during the rising movement, though not thick, might become protected by fresh accumulations, and thus be preserved for an extremely long period,— a consideration which I formerly overlooked.

Mr. Hopkins, in commenting on this subject, states that he believes the entire destruction of any sedimentary bed of considerable horizontal extent to have been of rare occurrence. ↑ My remarks apply solely to beds rich in fossils: I have admitted that sediment accumulated in extremely thick, solid, or extensive masses would escape denudation. The point in question is, whether widely extended formations, rich in fossils, and of sufficient thickness to last for a long period, would be formed except during periods of subsidence? My impression is that this has rarely been the case. As the subject of complete denudation has been broached by Mr. Hopkins, I may remark that all geologists, excepting the few who believe that they see in the metamorphic schists and plutonic rocks the heated primordial nucleus of the globe, will probably admit that rocks of this nature must have been largely denuded. For it is scarcely possible that these rocks should have been solidified and crystallized in a naked condition; but if the metamorphic action occurred at profound depths of the ocean, the former mantle may not have been thick. Admitting then that such rocks as gneiss, mica-schist, granite, diorite, &c, were once necessarily covered up, how can we account for the extensive and naked areas of such rocks in many parts of the world, except on the belief that they have subsequently been

deposits. In the case of an extensive and shallow sea, such as that within a large part of the Malay Archipelago, where the depth varies from 30 or 40 to 60 fathoms, a widely extended formation might be formed during a period of elevation, and yet not suffer excessively from denudation during its slow upheaval; but the thickness of the formation could not be great, for owing to the elevatory movement it would be less than the depth in which it was formed; nor would the deposit ... be much consolidated, nor .. be capped by overlying formations, so that it would run a good chance of being worn away by atmospheric degradation and by the action of the sea during subsequent oscillations of level. It has, however, been suggested by Mr. Hopkins, that if one part of the area, after rising and before being denuded, subsided, the deposit formed during the rising movement, though not thick, might afterwards become protected by fresh accumulations, and thus be preserved for a long period.

Mr. Hopkins also expresses his belief that sedimentary beds of considerable horizontal extent .. have rarely been completely destroyed. But all geologists, excepting the few who believe that our present metamorphic schists and plutonic rocks once formed the primordial nucleus of the globe, will probably admit that rocks of this nature must have been denuded on an enormous scale. ↑ For it is scarcely possible that these rocks could have been solidified and crystallized whilst uncovered; but if the metamorphic action occurred at profound depths of the ocean, the former protecting mantle may not have been very thick. Admitting then that such rocks as gneiss, mica-schist, granite, diorite, &c., were once necessarily covered up, how can we account for the naked and extensive areas of such rocks in many parts of the world, except on the belief that they have subsequently been