Mina Negress and Child.
May 26th.—Tijuca. In the pleasant environs of Rio there is no resort more frequented than the establishment of Mr. Bennett at Tijuca, and we were not sorry the day before yesterday to leave the hot, dusty city, with a pleasant party of friends, for this cluster of mountains, some eighteen hundred feet above the sea level and about eight miles from Rio. It takes its name from the peak of Tijuca, so conspicuous an object in the coast range. On our arrival we were very cordially welcomed by our host himself, who was not quite a stranger to us, for Mr. Agassiz has been already indebted to him for valuable collections. Mr. Bennett has an Englishman’s love of nature, and is very familiar with the botany and zoölogy of the beautiful region which has been his home for many years. Under his guidance, we have taken a number of pleasant rambles and rides, regretting only that we cannot avail ourselves for a longer time of his intimate knowledge of the locality and its productions.
I have alluded before to the perplexing character of the geology, and the almost universal decomposition of the rock surfaces, making it difficult to decipher them. The presence of the drift phenomena, so universal in the Northern hemisphere, has been denied here; but, in his long walk to-day, Mr. Agassiz has had an opportunity of observing a great number of erratic boulders, having no connection with the rocks in place, and also a sheet of drift studded with boulders and resting above the partially stratified metamorphic rock in immediate contact with it. I introduce here a letter written by him to his friend, Professor Peirce of Harvard University, under the first impression of the day’s experience, which will best explain his view of the subject.
“May 27th, 1865, Tijuca.
“My dear Peirce:—
“Yesterday was one of the happiest days of my life, and I want to share it with you. Here I am at Tijuca, a cluster of hills, about eighteen hundred feet high and some seven or eight miles from Rio, in a charming cottage-like hotel, from the terrace of which you see a drift hill with innumerable erratic boulders, as characteristic as any I have ever seen in New England. I had before seen sundry unmistakable traces of drift, but there was everywhere connected with the drift itself such an amount of decomposed rocks of various kinds, that, though I could see the drift and distinguish it from the decomposed primary rocks in place, on account of my familiarity with that kind of deposits, yet I could probably never have satisfied anybody else that there is here an equivalent of the Northern drift, had I not found yesterday, near Bennett’s hotel at Tijuca, the most palpable superposition of drift and decomposed rocks, with a distinct line of demarcation between the two, of which I shall secure a good photograph. This locality afforded me at once an opportunity of contrasting the decomposed rocks which form a characteristic feature of the whole country (as far as I have yet seen it) with the superincumbent drift, and of making myself familiar with the peculiarities of both deposits; so that I trust I shall be able hereafter to distinguish both, whether they are in contact with one another or found separately. These decomposed rocks are quite a new feature to me in the structure of the country. Imagine granite, gneiss, mica slate, clay slate, and in fact all the various kinds of rocks usually found in old metamorphic formations, reduced to the condition of a soft paste, exhibiting all the mineralogical elements of the rocks, as they may have been before they were decomposed, but now completely disintegrated and resting side by side, as if they had been accumulated artificially in the manner you have seen glass cylinders filled with variously colored sands or clays to imitate the appearance of the beds of Gay-Head. And through this loose mass there run, here and there, larger or smaller dikes of quartz-rock or of granite or other rocks equally disintegrated; but they retain the arrangement of their materials, showing them to be disintegrated dikes in large disintegrated masses of rock; the whole passing unmistakably to rocks of the same kind in which the decomposition or disintegration is only partial, or no trace of it visible, and the whole mass exhibiting then the appearance of an ordinary metamorphic set of rocks.
“That such masses forming everywhere the surface of the country should be a great obstacle to the study of the erratic phenomena is at once plain, and I do not therefore wonder that those who seem familiar with the country should now entertain the idea that the surface rocks are everywhere decomposed, and that there is no erratic formation or drift here. But upon close examination it is easy to perceive that, while the decomposed rocks consist of small particles of the primitive rocks which they represent, with their dikes and all other characteristic features, there is not a trace of larger or smaller boulders in them; while the superincumbent drift, consisting of a similar paste, does not show the slightest sign of the indistinct stratification characteristic of the decomposed metamorphic rocks below it, nor any of the decomposed dikes, but is full of various kinds of boulders of various dimensions. I have not yet traced the boulders to their origin; but the majority consist of a kind of greenstone composed of equal amounts of a greenish black hornblende and feldspar. In Entre Rios on the Parahyba, I was told by an engineer on the road that in Minas Geräes iron mines are worked in a rock like these boulders. This week I propose to explore the Serra da Mantiqueira,[[32]] which separates the province of Rio from Minas, and may advance the question further. But you see that I need not go to the Andes to find erratics, though it may yet be necessary for me to go, in order to trace the evidence of glacier action in the accumulation of this drift; for you will notice that I have only given you the evidence of extensive accumulations of drift similar in its characteristics to Northern drift. But I have not yet seen a trace of glacial action properly speaking, if polished surfaces and scratches and furrows are especially to be considered as such.
“The decomposition of the surface rocks to the extent to which it takes place here is very remarkable, and points to a new geological agency, thus far not discussed in our geological theories. It is obvious here (and to-day with the pouring rain which keeps me in doors I have satisfactory evidence of it) that the warm rains falling upon the heated soil must have a very powerful action in accelerating the decomposition of rocks. It is like torrents of hot water falling for ages in succession upon hot stones. Think of the effect, and, instead of wondering at the large amount of decomposed rocks which you meet everywhere, you will be surprised that there are any rocks left in their primitive condition. It is, however, the fact, that all the rocks you see are encased, as it were, in a lining of the decomposed part of their surface; they are actually covered with a rotten crust of their own substance.
“Ever truly yours,