GEOLOGY. 269 



with a structure exactly similar to the lacunar spaces intervening 

 between the outside of the proper walls of the chambers and the 

 intermediate skeleton, by which they become overgrown. He 

 stated that, even if the remarkable dendritic passages hollowed out 

 in the calcareous layers, and the arrangements of the minerals in 

 the Eozoic limestone, could be accounted for bv inorganic agen- 

 cies, there still remains the nummuline structure of the chamber 

 walls, to which no parallel can be shown in any undoubted min- 

 ei'al product. — Popular Science Review, April, 1866. 



DRIFT IN BRAZIL. 



According to Prof. Agassiz, as reported by his son in *' Silli- 

 man's Journal," for Nov., 1865, at Tijuca, a cluster of hills about 

 eighteen hundred feet high, and about seven or eight miles from 

 Rio, there is a drift hill with innumerable erratic boulders, as 

 characteristic as any ever seen in New England. He had before 

 seen unmistakable traces of drift in the Province of Rio and in 

 Minas Geraes ; but there was everywhei'e connected with the 

 drift itself such an amount of decomposed rocks of various kinds, 

 that it would have been difficult to satisfy any one not familiar 

 with drift, that thei'e is here an equivalent of the Northern drift. 

 There is found at Tijuca the most palpable superposition of drift 

 and of decomposed rocks, with a distinct line of demarcatioa 

 between the two. 



This locality afforded an opportunity of contrasting the decom- 

 posed rocks, which form a characteristic feature of the whole 

 country, with the superincumbent drift, so as to be able hereafter 

 to distinguish both, whether found in contact or separately. 

 These decomposed rocks are quite a new feature in the structure of 

 the surface of the country. Gi-anite, gneiss, mica slate, clay slate, in 

 fact all the various kinds of rocks usually found in old metamorphic 

 formations, are reduced to the condition of a soft paste, exhibiting 

 all the mineralogical elements of the rocks as they may have been 

 before they were decomposed, but now completely disintegrated, 

 and resting side by side, as if they had been accumulated artifi- 

 cially. Through this loose mass there were here and tliere larger 

 or smaller veins of quartz rock, or of granite or other rocks, equally 

 disintegrated ; but they retain the arrangement of their materials, 

 showing them to be disintegrated veins in large disintegrated 

 masses of rocks. The whole passes unmistakably to rocks of the 

 same kind, in which the decomposition or disintegration is only par- 

 tial, or no trace of it is visible ; and the whole mass exhibits, then, 

 the appearance of a set of oi'dinary metamorphic rocks. It is plain 

 that such masses forming everywhere the surface of the country 

 must be a great obstacle to the study of erratic phenomena ; and 

 it is not wonderful that those who seem familiar with the countiy 

 should entertain the idea that the surface rocks are everywhere 

 decomposed, and that there is no erratic formation or drift here. 

 But, upon close examination, it is easy to see that, while the de- 

 comjiosed rocks consist of the small ijarticles of the primitive 

 rocks, which they represent, with their veins and all other charac- 

 22* 



