“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,