Editorial. 



It is important to geologists to know to what extent the 

 glacial drift of northern latitudes owes its thickness to the secu- 

 lar decay of the rocks, and also whether we can use, in high lati- 

 tudes where decomposed rock material has been removed by ice, 

 data collected on the subject of rock decomposition in the tropics 

 where it has not been so disturbed. In the paper by Professor 

 Derby in the present number, he is disposed to take a conserv- 

 ative view of the subject of rock decomposition in Brazil ; and 

 Professor Derby's long residence in that country entitles his 

 opinion to much weight. 



Our studies of the subject of rock decay in Brazil were begun 

 and carried on under the impression, probably received from the 

 writings of Agassiz, Darwin, and others, that it was extraordinary 

 and the paper on the "Decomposition of Rocks in Brazil" cited 

 by Derby was given as a record of the facts and an attempt to 

 explain them. It is but just to confess, however, that when the 

 results were brought together we were not as much impressed by 

 them as we had expected to be. And the more we see of decay 

 in temperate regions, especially in unglaciated regions of grani- 

 toid rocks, the less striking does the decay of rocks in Brazil 

 appear. In California, in Virginia, in the great kaolin pits of St. 

 Yrieux, France, and in the Guadarrama Mountains of Spain we 

 have seen many examples of the decay of granites and gneisses 

 that are very nearly as deep as any we have seen in Brazil. 



The unequal decay of rocks, and even its absence, was not 

 overlooked in the article referred to by Derby. Mention should 

 have been made at that place of a letter by Dr. A. R. C. Selwyn, 

 published in the Geological Magazine (1877, p. 94) where he calls 

 attention to this unequal decay of rocks in Brazil and Australia 

 as a possible explanation of glacial lake basins in the north. 



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