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[in Darwin's hand:] Communication from M. Lund to the Academy at Copenhagen

The plains, the vallies, & the lower hills, are without exception overspread with a vast stratum of loose soil, from which the higher ranges alone are free. This stratum with a certain degree of uniformity combines no inconsiderable variety, which is shown partly in its subordinate beds, & partly in the occasional appearance of these beds, without any connection sequence by the side of each other. The most common form which these formations assume is that of a stratum of red coarse clay, from 10 to 30 & more feet in thickness, in which there is no trace of stratification or other divisions. Occasionally one may follow this clay bed over considerable tracts, without observing any remarkable intermixture extraneous substance; but usually it contains, in greater or less quantities, 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. Intermixed 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, & it is out of this same soil that the natives obtain gold & diamonds by washing. Quartz is also found in this bed of clay, under another form still more common in Province of Minas, viz that of sharp angular fragments

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grouped together so as to form regular beds, of from ½ to 1 ½ foot thick, & frequently of considerable extent. This siliceous stratum has no uniform depth in the clay, but often approaches the surface, & then forms the "Gr Campos" so characteristic of that country, & so unfriendly to agriculture, though clothed with so peculiar a vegetation. Amongst this quartz occur, although sparingly, fragments of other rocks, particularly of ironstone. The colour of the clay occasionally passes from red into dull ochre-yellow: & this change in colour is usually accompanied by a greater degree of fineness & uniformity of composition, with some slight trace of cleavage, or separation into cubical masses. I have hitherto had no opportunity of observing these beds of clay alternating with beds of sand; whereas this last formation appears on the surface over an extent of many degrees in length, in the broad vale watered by the St. Francisco river.

But the most peculiar character under which these newest formations present themselves in Brazil, is that called in this country "Tapanhoacanga," or negro-head. This formation is seen, like a stream of lava, spread over the plains, valleys, & hills, nay, even on the highest mountain-ridges of Brazil. It is most common in those districts where there are ironstone rocks of the older formations. It consists of a ferruginous conglomerate, composed of rolled & angular fragments of quartz, & other rocks of the country; but chiefly of the iron formation itself, such as ferriferous mica slate &c., united by a ferruginous cement of red, yellow, or black colour. Осcasionally, the fragments disappear, & the cementing substance alone remains. It forms a mass almost as hard as stone, with a smooth & often shining surface; inside, it is full of vescicular holes, which give it the appearance of a slag. It very often contains stone marrow, & in general the same minerals that form the constituent portions of the common red The origin of this quartz-bed was long a puzzle to me, until more careful examinations of the principal mountain range enlightened me on the subject. One of the most important members in the composition of that range is a vast mass of tale slate, with numerous veins & subordinate beds of quartz. This Talk slate, in most places, & to considerable depths, is in such a state of decomposition, as to crumble between the fingers, & the quartz separates with the touch into larger or smaller angular fragments. If we examine the beds of diluvium at the foot of this mountain chain, we there see the red homogeneous clay replaced by a mass of nothing but fragments of tale slate, lying in all directions, but containing also the same siliceous layers disposed with more regularity, so as often to look like connected quartz

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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. Occasionally 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. Intermixed 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; & it is out of this same soil that the natives obtain gold & 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, & frequently of considerable extent. This siliceous stratum has no uniform depth in the clay, but often approaches the surface, & then forms the 'Gruns Campos' so characteristic of that country, & so unfriendly to agriculture, though clothed with so peculiar a vegetation. Amongst this quartz occur, although sparingly, fragments of other rocks, particularly of ironstone. The colour of the clay occasionally passes from red into dull ochre-yellow: & this change in colour is usually accompanied by a greater degree of fineness & uniformity of composition, with some slight trace of cleavage, or separation into cubical masses. I have hitherto had no opportunity of observing these beds of clay alternating with beds of sand; whereas this last formation appears on the surface over an extent of many degrees in length, in the broad vale watered by the St. Francisco river.

But the most peculiar character under which these newest formations present themselves in Brazil, is that called in this country "Tapanhoacanga," or negro-head. This formation is seen, like a stream of lava, spread over the plains, valleys, and hills, nay, even on the highest mountain-ridges of Brazil. It is most common in those districts where there are ironstone rocks of the older formations. It consists of a ferruginous conglomerate, composed of rolled

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& angular fragments of quartz, & other rocks of the country; but chiefly of the iron formation itself, such as ferriferous mica slate &c., united by a ferruginous cement of red, yellow, or black colour. Осcasionally, the fragments disappear, & the cementing substance alone remains. It forms a mass almost as hard as stone, with a smooth & often shining surface; inside, it is full of vescicular holes, which give it the appearance of a slag. It very often contains stone marrow, & in general the same minerals that form the constituent portions of the common red

The origin of this quartz-bed was long a puzzle to me, until more careful examinations of the principal mountain range enlightened me on the subject. One of the most important members in the composition of that range is a vast mass of talc slate, with numerous veins & subordinate beds of quartz. This talc slate, in most places, & to considerable depths, is in such a state of decomposition, as to crumble between the fingers, & the quartz separates with the touch into larger or smaller angular fragments. If we examine the beds of diluvium at the foot of this mountain chain, we there see the red homogeneous clay replaced by a mass of nothing but fragments of tale slate, lying in all directions, but containing also the same siliceous layers disposed with more regularity, so as often to look like connected quartz beds. That this tale slate is in a secondary position, is evident from its irregular stratification; & if we follow its development from the foot of the mountain, it is easy to convince ourselves of its gradual passage & change into the common red clay, with its usual siliceous bed.

clay, particularly gold, which is often in sufficient quantity to be worked. The same conglomerate is likewise found in the caves, & contains the same animal remains as the red clay; so that there can be no doubt of the contemporaneousness of their formation.—