GEOLOGY. 247 



age, he refers the horizontal beds of clays and sandstones of the 

 Jequitinhonha and Sao Francisco valley, which lie undisturbed on 

 upheaved cretaceous rocks, and which are also covered by drift 

 clay. These beds must have been deposited when the continent 

 stood 3,000 feet lower than at present, the material being derived 

 from the decomposed gneissic rocks, and deposited rapidly in a 

 muddy sea unfit for animal life. 



The next formation, which is very wide-spread, is composed of a 

 layer of red clay, overlying as a general rule a layer of quartz 

 pebbles, varying in degree of firmness, and occasionally there are 

 intermingled fragments of gneiss, trap, or tertiary sandstone. 

 This sheet of structureless clays, gravels, and boulder deposits 

 stretching along: the whole coast, and covering alike the coast 



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tertiary plains, the elevated campus, and the semis from bottom 

 to top, is referred by Professor Agassiz to the drift, and in this 

 opinion he is sustained by Professor J[artt. All Rio de Janeiro, 

 and all the coast provinces visited by the latter, were thus covered. 

 It has been described in Minaes Geraes and Sao Paulo, and, 

 according to Professor Agassiz, it exists in various localities on the 

 coast, north of Pernarnbuco, and in the valley of the Amazons, 

 westward to the confines of Peru. In the province of Bahia 

 there are extensive bare, elevated rocky plains thickly strewn 

 with angular blocks of stone, some of which are erratics and 

 exactly resemble the drift-covered plains of the north. On similar 

 elevated plains, far removed from any higher land, Mr. J. A. 

 Allen (another member of the Thayer Expedition) found numer- 

 ous deep and smooth pot-holes worn in solid gneiss. They were 

 of various sizes, the largest seen being elliptical, 18 feet long by 

 10 wide and 27 feet deep. Similar pot-holes are known to be 

 formed by glacial water-falls, and they are found over the glaci- 

 ated regions of New Brunswick and Nova Scotia. Heaps of 

 debris, exactly resembling glacial moraines in the valley of 

 Tijuca near Rio, and Professor Agassiz has described others still 

 more perfect in Ceara, only 4 or 5 degrees south of the equator. 



Professor Hartt thus states his belief in regard to the drift 

 deposits: " I believe that during the time of the drift the country 

 stood at a much higher level than at present, and that it was cov- 

 ered by a general glacier. Over the coast region, where de- 

 composition of the rocks had largely obtained, and where the 

 surface of the rock, rendered even by this agent, had been covered 

 by a thick layer of loose material, the glacier reworked this 

 loose material, and when it disappeared left it as a paste, in 

 which the harder materials, such as fragments from quartz 

 veins, more or less rounded, were embedded. The layer 

 of quartz pebbles underlying the paste appears to have consisted 

 of coarser fragments borne along by the bottom of the glacier, 

 while the paste seems to have been more or less distributed 

 through the body of the glacier. In the drift paste I have never 

 seen organic remains of any kind." 



Mode of Occurrence of Diamonds. In speaking of the diamond- 

 washing at Pitanga,- Professor Hartt says : " The diamonds appear 

 tome to come from the tertiary beds of the neighboring hills: and 



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this seems to be the opinion of Mr. Nicolay, who shows that the 



