ed chiefly of
crystalline and metamorphic rocks. Some of the metamorphic rocks may
belong to the older Palaeozoic period, but the greater part of the
series is probably Archaean. Similar rocks cover a large area in the
province of Goyaz and in the south of the Matto Grosso, and they form,
also, the hills which border the basin of the Amazon on the confines
of Venezuela and Guiana. They constitute, in fact, an incomplete rim
around the basin of sedimentary beds which occupies the Amazonian
depression. In a large part of this basin the covering of sedimentary
deposits is comparatively thin. The crystalline floor is exposed in
the valleys of the Madeira, Xingu, &c. Some of the rocks thus exposed
are, however, eruptive (e.g. in the Tapajoz), and probably do not
belong to the Archaean. The crystalline rocks are succeeded by beds
which have been referred to the Cambrian and Silurian systems. In the
valley of the Trombetas, one of the northern tributaries of the
Amazon, fossils have been found which indicate either the top of the
Ordovician or the bottom of the Silurian. In the Maecuru, another
northern affluent, graptolites of Ordovician age have been discovered,
and Silurian fossils are said to have been found in the Maraca.
Elsewhere the identification of the Silurian and older systems does
not rest on palaeontological evidence. Devonian beds cover a much more
extensive area. They crop out in a band some 25 to 50 m. north of the
lower Amazon and in another band at a still greater distance south of
that river. These bands are often concealed by more recent deposits,
but it is clear that in this region the Devonian beds form a basin or
synclinal with the Amazon for its axis. Devonian beds also lie upon
the older rocks in the Matto Grosso and other provinces in the
interior of Brazil, where they generally form plateaux of nearly
horizontal strata. Fossils have been found in many localities. They
belong to either the lower or the middle division of the Devonian
system. The fauna shows striking analogies with that of the Bokkeveld
beds of South Africa on the one hand and of the Hamilton group of
North America on the other. The Carboniferous system in Brazil
presents itself under two facies, the one marine and the other
terrestrial. In the basin of the Lower Amazon the Carboniferous beds
lie within the Devonian synclinal and crop out on both sides of the
river