400 A JOURNEY IN BRAZIL. 
with the rock in place ; and also a bluff of this super- 
ficial deposit studded with boulders, resting above the 
partially stratified metamorphic rock.* Other excellent 
opportunities for observing this formation, also within 
easy reach from the city, are afforded along the whole 
line of the Dom Pedro Segundo Railroad, where the 
cuts expose admirable sections, showing the red, unstrat- 
ified, homogeneous mass of sandy clay resting above the 
solid rock, and often divided from it by a thin b'ed 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 con- 
tinent. 
It is true that the extensive decomposition of the un- 
derlying 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 puz- 
zling 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 ap- 
pearances aright ; and I may say that I have learned to 
recognize everywhere the limit between the two forma- 
tions. There is indeed one safe guide, namely, the un- 
* See Chapter III. p. 86. 
