92 HISTORY OF THE EARTH 
and is sometimes separated from them by a thin layer of sandstone. 
Galleries have been driven under the basalt for the purpose of pro- 
curing ore from the subjacent formations, affording the means of 
ascertaining beyond all doubt, that it cannot have been thrown up 
from below. Werner finding it lying in some cases upon a rock 
evidently secondary, and seeing upon the spot no evidence of its 
igneous origin, classed it with his secondary rocks that have been 
deposited from water.. The correctness of this classification was 
questioned by other geologists, and the disciples of Werner hold- 
ing themselves bound to maintain the positions of their master, 
there arose a long and angry dispute respecting the origin of ba- 
salt, and in general of the floetz or secondary trap rocks. 
Secondary trap formations are seldom extensive. The rocks of 
this class do not constitute great mountain chains. They general- 
ly occupy an unconformable and overlying position on the top of 
other strata. Sometimes they are irregular shapeless masses, but 
they occur also in tables and of a globular form. A large block 
of basalt is often divided by fissures into prisms, the numberof the 
sides of which is variable, from three to nine. Four or five sides 
are the most common. The sides of a trap formation frequently 
present high perpendicular precipices, or a succession of these, of 
less elevation, creating in the latter case natural terraces, whence the 
name of Trap Rocks from the Swedish, trappa, signifying a stair. 
The neighborhood of Edinburgh, the Western Islands of Scot- 
land, the North of Ireland, and the Northern States furnish ex- 
amples. 
The igneous origin of the trap rocks has been inferred from 
a variety of facts and observations, some of which are here 
stated. 
1. Basalt and some varieties of compact lava very much re- 
semble each other. They have the same texture, color, and ap- 
pearance of having been subjected to the action of fire. That 
variety of basalt on the other hand, which bears the name of 
Amygdaloid, imitates very exactly porous lava. After having 
been exposed to an intense heat, basalt and lava if suddenly cool- 
ed, assume the characters of glass, if slowly cooled, of stone. 
The general range of their characters, both physical and chemi- 
cal, is much the same. Basalt passes by insensible gradations 
into the other trap rocks, so lhat whatever be the opinion that 
is entertained respecting the one, it must be held in regard to 
the others. 
2. The position of the trap rocks, with respect to other rocks 
and strata, is generally that which a mass of matter would assume 
if poured in a fluid or semi-fluid state from a volcano. The su- 
perincumbent and overlying position is peculiarly appropriate to 
rocks that have been formed in this way. 
3. Basaltic rocks are most frequently met with in those coun- 
tries which exhibit other proofs of having been the seats of vol- 
canic action. Daubisson when he had seen only the basalts of 
