121 
THE EARTH 5 S CRUST, 
SECTION OF THE EARTH’S CRUST. 
§ 85. This section is designed to illustrate the structure of 
the earth’s crust at the parallel of 40° or 45° N. If a segment 
of the earth were cut off at this parallel, we may suppose the 
lower part of the consolidated face to consist of pyrocrystalline 
rocks intersected by veins of the same material, but of a date pos¬ 
terior to the original consolidated matter. These veins consist of 
the newer granites, dykes of trap, serpentine, veins of iron, 
copper, lead, &c.; and as there is no country which has been 
explored, which does not furnish clusters of veins and dykes ot 
some kind, we may regard the crust as constituted of pyrocrys¬ 
talline rocks, penetrated everywhere, and traversed by a net¬ 
work of veins. As Vesuvius, Etna, and South American vol¬ 
canoes are hut ejected matter in beds, which are fissured and 
dyked in every direction, so the general crust may be regarded 
as equally fissured and traversed by the more recent of the 
erupted rocks in the form of veins and dykes. It has been 
stated in the preliminary remarks, that the bottom of the ocean 
is not a plane, but an irregular surface of the same character 
as the dry land—sinking in places to the most profound depths, 
and rising again in mountain peaks, some of which reach the 
ocean’s level, while others peer just above it. A ll those parts of 
the earth’s crust which lie beneath the area, represented as Ame¬ 
rica, Europe, and Asia, were once beneath the oceans. The vast 
amount of sediments -which are accumulated upon these conti¬ 
nents show the vastness of the time during which they were 
covered with water; and as the extreme height of the mountains 
of these continents nearly equal the profoundest depths of the 
oceans, the vertical exposure of the crust, or the depths to which 
our observations may extend, rather exceed 60,000 feet. 
The only points to which I wish to call the attention of the 
student, on the physical map of the world, are the general direc¬ 
tions of the old and new continents, both of which were deter¬ 
mined by the direction of the upheaval of their principal moun- 
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