318 Scientific Intelligence. 



the kinds of rocks, and its earth-making results. Some of the 

 methods of decay are also considered, especially its dependence 

 on the carbonic and organic acids in the soil and on the moisture 

 and warmth of the climate. The great contrast in depth of 

 decomposition between the northern and southern States is dwelt 

 on ; and besides explaining this difference by reference to glacial 

 removal in one region and not in the other, and to difference in 

 climate, he refers to the recent elevation in the former region as 

 having quickened erosion and removal by transportation. The 

 author, moreover, sustains the commonly accepted opinion that 

 subaerial decay is still going on as in the past, with like rate of 

 progress. He also dwells on the prevailing red color of the earth 

 and clays resulting from the decay — terra rossa, as a wide-spread 

 deposit is called in Europe. Finally, the red color of red sand- 

 stones is attributed to the red iron oxide produced by the 

 decomposition or decay, and not to any subsequent change in the 

 history of the deposits. 



The contrast in colors between the northern and southern 

 States is spoken of as "a contrast between a glaciated country 

 and a region in which atmospheric decay has progressed uninter- 

 ruptedly for ages." Mr. Russell, knowing less of New England 

 than of other parts of the country, does not appreciate, as strongly 

 as Professor Crosby whom he criticizes, the full character of this 

 difference. There is wide decomposition at the north, and its 

 rapid progress in the case of syenites, mica schists, gneisses, 

 granites, and hornblende schists during the past forty years is 

 very strikingly exhibited alongside of many railroad cuts. The 

 fact to be accounted for is that these decompositions over New 

 Enorland, whether in the trap of trap dikes or in metamorpl^c 

 rocks, produces almost never red earth, while at the south red 

 earth predominates. The glacial movements and orographic 

 changes have nothing to do with this. The fact is simply that 

 in New England the result of the iron oxidation attending the 

 decay is limonite, the hydrous yellow-brown iron oxide, and 

 not anhydrous Fe 2 3 . The writer has often tried to discover a 

 reason for the different result at the south ; he does not find one 

 in Mr. Russell's excellent paper. The question was brought to 

 the writer's attention long since at the volcanic islands of the 

 Pacific (Samoa among them), where the larger amount of rock 

 decay (as about the East Maui Mountain), produces yellow-brown 

 and reddish brown earths, while dry cinder cones (as in the 

 Maui crater), are often red. The decay of a granitoid gneiss a 

 dozen miles east of New Haven goes on rapidly through the oxi- 

 dation of the iron of the mica ; and it makes the rock brown and 

 comparatively weak to a depth in some places of six inches to a 

 yard, with very little attendant decay of the feldspar. Further, 

 the great blocks thus deeply browned have often a white exterior 

 two inches or so deep which had been blanched by surface rain- 

 waters, perhaps with the aid of carbonic or organic acids, and 

 this white crust portion is the granitoid rock with the mica gone. 



