hole line of the Railroad of Dom Pedro Segundo,
where the cuts expose admirable sections, showing the red, unstratified,
homogeneous mass of sandy clay resting above the solid rock, and often
divided from it by a thin bed of pebbles. There can be no doubt, in the
mind of any one familiar with similar facts observed in other parts of
the world, that this is one of the many forms of drift connected with
glacial action. I was, however, far from anticipating, when I first met
it in the neighborhood of Rio, that I should afterwards find it
spreading over the surface of the country, from north to south and from
east to west, with a continuity which gives legible connection to the
whole geological history of the continent.

It is true that the extensive decomposition of the underlying rock,
penetrating sometimes to a considerable depth, makes it often difficult
to distinguish between it and the drift; and the problem is made still
more puzzling by the fact that the surface of the drift, when baked by
exposure to the hot sun, often assumes the appearance of decomposed
rock, so that great care is required for a correct interpretation of the
facts. A little practice, however, trains the eye to read these
appearances aright, and I may say that I have learned to recognize
everywhere the limit between the two formations. There is indeed one
safe guide, namely, the undulating line, reminding one of _roches
moutonnees_,[C] and marking the irregular surface of the rock on which
the drift was accumulated; whatever modifications the one or the other
may have undergone, this line seems never to disappear. Another
deceptive feature, arising from the frequent disintegration of the rocks
and from the brittle character of some of them, is the presence of loose
fragments, which simulate erratic boulders, but are in fact only
detached masses of the rock in place. A careful examination of their
structure, however, will at once show the geologist whether they belong
where they are found, or have been brought from a distance to their
present resting-place.

But while the features to which I have alluded are unquestionably drift
phenomena, they present in their wider extension, and especially in the
northern part of Brazil, as will hereafter be seen, some phases of
glacial action hitherto unobserved. Just as the investigation of the ice
period in the United States has shown us that ice-fields may move over
open level plains, as well as along the s