INDUSTRIAL PROGRESS DURING THE YEAR 1873. xlix 



clays thus produced were derived the argillaceous rocks of 

 the various geological periods up to the present, while the 

 separated quartz and the dissolved and precipitated silica 

 liave formed the various silicious rocks, sandstones, quartzites, 

 and cherts. To the alkaline and earthy carbonates produced 

 in this process of decay are due the limestones and dolomites 

 of the paleozoic sea ; while the iron dissolved out by organic 

 agency from the decayed materials has furnished not only the 

 deposits of iron in various forms which appear at different 

 horizons in the paleozoic series, but also the great accumula- 

 tions of limonite ores found along: the base of the Blue Rido^e. 

 Some of these have been directly derived from the decom- 

 posed rocks, while others are due to secondary changes in the 

 paleozoic iron-bearing strata. 



This decay of the crystalline rocks was, in his opinion, uni- 

 versal, but the softened materials to the north and east have 

 been removed by aqueous and, in some cases, by glacial ero- 

 sion from the surface, leaving bare the hard, unchanged por- 

 tions. As late as the Miocene, he conceives that the hills of 

 New England were covered with decayed materials like the 

 Blue Ridge of to-day, and from these came deposits of clays 

 and ores like those of Brandon, Vermont, which are vestiges 

 of formations that w'ere swept away during the great sub- 

 mergence at the close of the Pliocene; since which time no 

 considerable decomposition has gone on, as is clear from the 

 preservation of the glacial scratches. He, however, supposes 

 this process of decay to have been continuous up to a com- 

 paratively recent period. As we go southward, where ero- 

 sion was less active, we find the partially disintegrated por- 

 tions of the rocks undisturbed, and finally the completely 

 decomposed strata still in place, showing that this region has 

 not for long ages been exposed to erosion or denudation. 

 The permeability of this superficial coating, due to its peculiar 

 structure and its vegetable covering, has prevented its degra- 

 dation by atmospheric waters. This decay and disintegration 

 was, according to Dr. Hunt, a necessary preliminary to gla- 

 cial and erosive action, which removed already softened ma- 

 terials. Mr. Burbank has, in this connection, furnished an 

 important contribution to our notions of the superficial drift- 

 deposits, by showing that in some parts of North Caroli- 

 na, where the hardened crystalline granitic rocks are incom- 



3 



