The Serra de Araripe, or that which runs between Crato and Barra do Jardim, is only an eastern branch of an elevated table-land which stretches continuously from the sea-coast, southward, and forms a natural boundary between the two great provinces of Ceará and Piauhy. It is generally elevated from 500 to 1,000 feet above the level of the country to the east of it, but not so much above that to the west; to this range the name of Serra Vermelha is given by the Portuguese, and Ibiapaba by the Indians. Between the 10th and 11th degrees of latitude it takes a westerly direction, and in about 47° of longitude takes a northerly sweep, finally terminating at the mouth of the River Amazon, under the equator, the country which it surrounds forming a valley of great extent, including the entire provinces of Piauhy and Maranham. This elevated range varies much in breadth, as many branches run off from it, both to the east and to the west; the top is nearly perfectly level, forming, as before mentioned, what the Brazilians call Taboleiras. The great mass of the Serra consists of a very soft, whitish, yellowish, or reddish-coloured sandstone, which in many places must be more than six hundred feet thick; and in this rock exist the nodules which contain the fossil fishes. The circumstance that first led me to suspect this rock belonged to the chalk formation, was an immense accumulation of flints and septaria similar to those of the chalk of England, which I found on the acclivity of the range during a journey made along its base to the north of Crato. I now began to inquire if anything like chalk was found in the neighbourhood, when I learned there were several pits in the Serra, whence the inhabitants obtained it for the purpose of white-washing their houses; these pits I afterwards found to be situated in a deep layer of red-coloured diluvial clay, which lies immediately over the sandstone of the Serra. In a ravine near Crato I endeavoured to ascertain the formation on which the sandstone rested, when I found it to consist of several layers of more or less compact limestones and marls, with a bed of lignite about two feet thick; upon what these rested I could not at that time ascertain, but some time afterwards when I crossed to the west side of the range, I found these limestones existing upon a deposit of very dark-red coarse-grained sandstone, abounding in small nodules of iron-stone. Thus we find that the structure of the rocks in this locality is very similar to that of the chalk formation in England; there is
1st. A ferruginous sandstone deposit, equivalent to the lower green sand or Shanklin sand.
2nd. A deposit of marls, soft and compact limestones, and lignite, equivalent to the English gault.
3rd. A very thick deposit of fine-grained, soft, variously coloured sandstone, containing Ichthyolites, equivalent to the upper green sand of England.
4th. The white chalk itself, and flints occurring in pits partially covered by red diluvial clay.
Flints are very common along the foot of the Serra, to the N.W. of Crato, but none were found in any of the chalk-pits that I examined: I learned, however, that at a considerable distance to the north of Crato, at a portion of this mountain range, called the Serra de Botarité, both chalk and flints are far more abundant than they are near the former place, where they seem to have been almost entirely washed away, previous to the deposition of the red clay in which they are now found.
Since the time when these rocks were first deposited at the bottom of the sea to the present period, both they and the surrounding country must have undergone various changes with respect to elevation; but before making any observations on this subject, I will point out the various places where I have met with traces of the chalk formation, besides that just described. In 1838, during my voyage up the Rio de San Francisco, which empties itself into the Atlantic between the 10th and 11th degrees of south latitude, I obtained specimens of the rock on which the Villa do Penêdo is built, and on comparison these proved to be identical with those from the upper sandstone of Crato. In 1839, I found the ferruginous sandstone of Crato extending westward thence about 500 miles, and in the year 1841 I observed at Maranham, in 2° of south latitude, and 44° of west longitude, a formation very similar to that at Crato. The whole island on which the city of Maranham is built, consists of a very dark-red ferruginous sandstone; on the main land to the westward, the same rock was observed rising a little above the sea level, but immediately upon it there exists a deposit, in some places more than 50 feet thick, of a yellowish and greenish coloured sandstone, very soft, and of a marly nature.
From these data, then, I think there can be little doubt that the whole of that immense shoulder which forms the more easterly point of the American continent, has at one time been a great depository for the chalk formation. The only other rocks that I observed in places denuded of the deposits belonging to the chalk are, 1st, gneiss and mica-slate, the layers of which crop out in nearly a vertical direction, as was frequently observed on my journey from the coast, and during my voyage up the Rio de San Francisco; and, 2nd, beds of grey-coloured clay-slate, which I passed over about 18 leagues below Crato. The whitish coarse-grained sandstone that I met with immediately afterwards, is probably equivalent to the ferruginous sandstone found on the west side of the range; from this it would appear, that between the cretaceous series and the primary stratified rocks, there are no traces either of the carboniferous or the oolite formations, nor in any part of Brazil through which I afterwards travelled did I meet with any signs of them.[5]
We have already seen that the country, from the coast to Crato, is for the most part level, large portions being covered with coarse white sand or gravel of various sizes, which give it the appearance of the dried up bed of an immense river; much of this gravel consists of flints, and intermingled with them are numerous boulders of various sizes, more or less rounded, consisting of granite, gneiss, and quartz. Whenever these gravelly tracts cease to appear, the surface of the country is covered with a deposit of the same kind of red clay which lies over the upper sandstone of the table-land. To the westward of this table-land, considerable portions are covered with the variously shaped iron-stone nodules, found in the ferruginous sandstone, and which have accumulated from the decay of that rock.
I have now to offer a few remarks on the changes of elevation which this part of the continent has undergone since the chalk rocks were first deposited; it is manifest that that deposition took place at the bottom of a shallow ocean, and it admits of no doubt that at some subsequent period it has been gradually elevated above the level of the sea; it is evident that this elevation has been gradual, from the horizontal position of the strata of which the deposit is formed; for had the elevating cause been sudden and violent, their original position would not have been so perfectly maintained. The first portion that emerged from the sea was probably the long elevated table-land, which for a period must have formed a neck of land separating the Atlantic Ocean on the east, from the great bay which the immense valley to the westward must then have formed.