8 MUSEUM OF COMPARATIVE ZO-OLOGY. [Jan. 



taries. The ichthyological collections will undoubtedly be the 

 main feature of the expedition, and the inhabitants of the 

 various hydrographic basins will be collected in such a thor- 

 ough manner that it is hoped some light will be thrown on the 

 geographical distribution of fishes, taking the different fresh 

 water basins of Brazil as our basis. 



The erratic phenomena have also been most successfully 

 observed in the vicinity of Rio Janeiro, especially at Tijuca, 

 a cluster of hills about 1,800 feet high and about seven miles 

 from Rio, where a drift-hill is found with innumerable erratic 

 boulders, as characteristic as any ever seen in New England. 

 Professor Agassiz had already observed unmistakable traces of 

 drift in the province of Rio and in Minas Geraes, but, everywhere 

 connected with the drift itself, was such an amount of decom- 

 posed rocks of various kinds that it would have been difficult 

 to satisfy any one not familiar with the drift that here was an 

 equivalent of the northern drift ; but there is found at Tijuca 

 the most palpable superposition of drift and of decomposed 

 rock, with a distinct line of demarcation 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 that hereafter any 

 one may distinguish them, whether found in contact or sepa- 

 rately. The decomposed rocks are quite a new feature in the 

 structure of the country, granite, gneiss, mica slate, clay slate, 

 in fact all the various kinds of rocks usually found in meta- 

 morphic formations, are reduced to the condition of a soft 

 paste, exhibiting all the mineralogical elements of the rocks as 

 they were before their decomposition ; but now completely disin- 

 tegrated and resting side by side as if accumulated artificially. 

 Through this loose mass there runs here and there larger or 

 smaller dykes of quartz rock, or of granite or other rocks, 

 equally disintegrated ; but they retain the arrangement of their 

 materials, showing them to be disintegrated dykes in large dis- 

 integrated masses of rock; the whole passing unmistakably 

 to rocks of the same kind in which the decomposition or disin- 

 tegration is only partial, or no trace of it visible, and the whole 

 mass exhibits then the appearance of an ordinary metamorphic 

 set of rocks. It is plain that such masses, forming everywhere 

 the surface of the country, must be a great obstacle to the 



