IfiSi Mr. Gardner on the Existence of Chalk in Brazil. 



We have already seen that the chalk formation at one time must have 

 covered a very great tract of the surrounding country ; and we may 

 very reasonably conclude that it was during the gradual elevation of 

 the land that the action of the waves of the ocean as gradually destroyed 

 the soft material of which it had been fabricated. But long after this 

 had been accomplished, and at a comparatively recent geological 

 period, the whole country seems again to have been covered with 

 water, — not only the comparatively level country between the shores 

 of the present sea, and the elevated table-land, but even the highest 

 parts of the table-land itself. This is proved by the thick covering 

 which exists on both of a deep red-coloured diluvial clay, similar to 

 that which I have observed to cover nearly the whole surface of Brazil, 

 from the sea shore to the summits nearly of the highest mountains, 

 and which is often more than forty feet in thickness. When this is 

 cut through it is found to consist of various layers of clay and gravel, 

 in which are embedded rounded stones of various sizes. These have 

 evidently been deposited from water ; and in that part of the country 

 of which we are now speaking, this deposition of clay must have taken 

 place at a period subsequent to the denudation of the country to the 

 east and west of the table-land. This could only have been accom- 

 plished by the sinking of the land again beneath the level of the sea ; 

 and this will also account for the nearly total destruction of the white 

 chalk, as well as for those small cones of it which remain embedded in 

 the red clay — that deposit having been laid down before the whole of 

 the chalk could be washed away. Since then, this part of the conti- 

 nent must have gradually emerged a second time from the bosom of 

 the ocean. 



No specimens of the rocks of this formation were sent home to 

 Mr. Bowman along with the fossil fishes ; but no sooner did M. Agassiz 

 see them, than from their zoological characters alone, he pronounced 

 them to belong to the chalk series. It is well known that this learned 

 naturalist divides all fishes into four great classes, from the nature of 

 their scales. Two of these, the Ctenoids and Cycloids, never make their 

 appearance in any of the rocks beneath the chalk, and it was from his 

 knowledge of this fact that he immediately decided my specimens 

 to be from that formation, they consisting principally of Ctenoids and 

 Cycloids. The fishes, as may be seen from the specimens, are in the 

 most perfect state of preservation, and are contained in an impure 

 fawn-coloured limestone. The blocks, however, in which they are 

 preserved, are only nodules contained in the yellowish coloured sand- 

 stone. They have in general somewhat the form of the imbedded fish, 

 and the carbonacious matter has apparently aggregated round it by 

 chemical attraction from the sandstone while in a soft state. These 

 nodules being harder than the sandstone, have by its gradual decay 

 accumulated at various places along the acclivity of the range, and I 

 possess specimens both from the east and the west side of it. The 



