To this latter cause is due, in great degree, the billowy, undulating character of the valleys. Another cause of difficulty in tracing the erratic phenomena consists in the number of detached fragments which have fallen from the neighboring heights. It is not always easy to distinguish these from the erratic boulders. But a number of localities exist, nevertheless, where the drift rests immediately above stratified rock, with the boulders protruding from it, the line of contact being perfectly distinct. It is a curious fact, that one may follow the drift everywhere in this region by the prosperous coffee plantations. Here as elsewhere ice has been the great fertilizer,—a gigantic plough grinding the rocks to powder and making a homogeneous soil in which the greatest variety of chemical elements are brought together from distant localities. So far as we have followed these phenomena in the provinces of Rio and Minas Geräes, the thriving coffee plantations are upon erratic drift, the poorer growth upon decomposed rock in place. Upon remarking this, we were told that the farmers who are familiar with the soil select that in which they find loose rocks imbedded, because it is the most fertile. They unconsciously seek the erratic drift. It may not be amiss to point out some of the localities in which these geological phenomena may be most readily studied, since they lie along the public road, and are easy of access. The drift is very evident in the swamp between Mauá and Raiz da Serra on the way to Petropolis. In ascending the Serra at the half-way house there is an excellent locality for observing drift and boulders; and beyond one may follow the drift up to the very top of the road. The whole tract between Villa Theresa and Petropolis is full of drift. Just outside of Petropolis, the Piabanha has excavated its bed in drift, while the banks have been ravined by the rains. At the station of Correio, in front of the building, is also an admirable opportunity for observing all the erratic phenomena, for here the drift, with large boulders interspersed throughout the mass, overlies the rock in place. A few steps to the north of the station Pedro do Rio there is another great accumulation of large boulders in drift. These are but a few of the localities where such facts may be observed.