38 OP THE OVERLYING AND VOLCANIC ROCKS. 
OF THE OVERLYING AND VOLCANIC ROCKS. 
23. In some parts of the world this is an important class, but 
as they occupy only a very limited space in our own country, 
our notice of them will be brief. It is well known that volca- 
noes pour out from their summits or their sides, a mass of melted 
matter which flows over the adjacent country and covers it with 
a bed of rock, sometimes compact, but more frequently more or 
less cellular, called lava. But besides proper lava, there is a 
considerable number of substances, some of them apparently 
very ancient, occupying like it an unconformable and overlying 
position upon the surface of the other strata, and so connected 
with it by a series of intervening rocks, that it is not doubted 
that they too have issued in a melted state from the interior of 
the earth. These rocks remain to be described. 
All the varieties of overling rock are found resting on recent 
secondary formations, than which they of course are more re- 
cent. They are called trap rocks, from the Swedish " trappa," 
signifying a stair. This name (which is not, however, applied 
by geologists with any great degree of precision) was given them 
with reference to the disposition many of them exhibit to assume 
a columnar shape and to divide into steps, forming natural terra- 
ces. The trap rocks often occur in veins, the intersection of 
which proves that there has been more than one formation of 
some or all of the varieties. They all consist of a paste, most 
commonly of the nature of feldspar, but sometimes of indurated 
clay, or of a substance intermediate between these two, coloured 
by hornblende or angite disseminated through its whole mass, or 
containing imbedded crystals of one of these substances, or of 
feldspar, constituting in the latter case a porphyry. 
1. When the rock does not differ essentially in its composition 
from feldspar, and is but slightly discoloured by foreign ingre- 
dients, having a gray and glassy aspect, it is called Trachyte. It 
abounds in the southern part of France and along the whole chain 
of the Andes. The summit of Chimborazo is formed altogether 
of this rock. 
2. When the rock has a granular structure and the feldspar is 
flesh coloured, and contains imbedded crystals of hornblende, it 
is proper syenite. But if there be a considerable admixture of 
the ingredients of syenite communicating to the mass a dark 
colour, it takes the name of greenstone, and if they be very in- 
timately blended, it is called basalt. 
8, A basalt containing almond shaped cavities filled with some 
foreign substance, as limestone or chalcedony, is amygdaloid or 
toadstone. 
4, When an uniform homogeneous base has small crystals of 
feldsdar imbedded in it in considerable numbers, it becomes por^ 
phyry, which is not however confined to the overlying rocks, 
But sometimes alternates with clay-slate. Porphyry abounds 
