PREVIOUS TO THE LAST GEOLOGICAL REVOLUTION. 3 



with the rounded forms assumed, frequently gives the rocky 

 masses the appearance of bronze statues. 



The number of caves already visited by me in Brazil, 

 amounts to eighty-eight, to the whole of which the charac- 

 ters here described are more or less applicable ; and these 

 characters are such as, in my opinion, leave no doubt as to 

 the mode of formation of the caves. In traversing them, one 

 appears to be walking by the rock-bound shore of ocean, and 

 to see its naked cliffs as they are hollowed and smoothed by 

 the waves. And doubtless, such is their origin ; doubtless, 

 we must assign the formation of these caverns to those peri- 

 ods when either this whole tract, that is now dry land, was 

 covered with vast lakes ; or when it yet rested in the bosom 

 of the sea. At any rate, it is certain that the filtration of wa- 

 ter through the limestone, is quite insufficient to explain the 

 phenomena we have been describing, and particularly the 

 deep yet blind holes gnawed into the roof 



All these caves are more or less filled with soil, which I 

 will take the liberty of describing, by giving a short account 

 of the newest formation that covers the surface in this part of 

 Brazil : it is exactly the same as that which is found inside 

 the caverns. 



The plains, the valleys, and the lower hills are, without 

 exception, overspread by a vast layer of loose soil, from which 

 the higher ranges alone are free. This stratum, with a cer- 

 tain degree of uniformity combines no inconsiderable variety, 

 which is partly shown in its subordinate beds, and partly in 

 the occasional appearance of these beds, without any such 

 sequence, by the side of each other. The most common form 

 assumed by these formations, is that of a stratum of coarse, 

 red clay, from ten to thirty or more feet in thickness, in which 

 there is no trace of stratification or other divisions. Occa- 

 sionally we may follow this clay -bed over considerable tracts, 

 without observing any remarkable extraneous substance ; but 

 it usually contains, in a greater or less quantity, rolled stones 

 of quartz, from the size of a pigeon's egg to that of a man's 

 head. These boulders are sometimes strewed about without 

 order ; sometimes they form more or less regular beds. In- 

 termixed with these rolled quartz stones are found, but in far 

 less quantity, similarly rolled fragments of the other kinds of 

 rock belonging to this district ; and it is out of this same soil 

 that the natives obtain gold and diamonds by washing. — 

 Quartz is also found in this bed of clay, under another form 

 still more common in the Province of Minas, namely, that of 

 sharp angular fragments, grouped together so as to form 

 regular beds, of from six to eighteen inches thick, and fre- 



