INTRUSION OF TRAP BETWEEN STRATA. 
517 
according to their composition, structure, and the fractures 
which they may have experienced, and perhaps, also, accord¬ 
ing to the quantity of water (so capable of being heated) 
which they contain. It must happen in some cases that the 
component materials are mixed in such proportions as to 
prepare them readily to enter into chemical union, and form 
new minerals; while in other cases the mass may be more 
homogeneous, or the proportions less adapted for such union. 
We must also take into consideration, that one fissure may 
be simply filled with lava, which may begin to cool from the 
first; whereas in other cases the fissure may give passage to 
a current of melted matter, which may ascend for days or 
months, feeding streams which are overflowing the country 
above, or being ejected in the shape of scoria© from some 
crater. If the walls of a rent, moreover, are heated by hot 
vapor before the lava rises, as we know may happen on the 
flanks of a volcano, the additional heat supplied by the dike 
and its gases will act more powerfully. 
Intrusion of Trap between Strata. —Masses of trap are not 
unfrequently met with intercalated between strata, and 
maintaining their parallelism to the planes of stratification 
throughout large areas. They must in some places have 
forced their way laterally between the divisions of the strata, 
a direction in which there would be the least resistance to 
an advancing fluid, if no vertical rents communicated with 
the surface, and a powerful hydrostatic pressure were caused 
by gases propelling the lava upward. 
Relation of Trappean Rocks to the Products of active Volca¬ 
noes. —When we reflect on the changes above described in 
the strata near their contact with trap dikes, and consider 
how complete is The analogy or often identity in composition 
and structure of the rocks called trappean and the lavas of 
active volcanoes, it seems difiicult at first to understand how 
so much doubt could have prevailed for half a century as to 
whether trap was of igneous or aqueous origin. To a cer¬ 
tain extent, however, there was a real distinction between 
the trappean formations and those to which the term volca¬ 
nic was almost exclusively confined. A large portion of 
the trappean rocks first studied in the north of Germany, and 
in Norway, France, Scotland, and other countries, were such 
as had been formed entirely under water, or had been inject¬ 
ed into fissures and intruded between strata, and which had 
never flowed out in the air, or over the bottom of a shallow 
sea. When these products, ‘therefore, of submarine or sub¬ 
terranean igneous action were contrasted with loose cones 
of scoriae, tuff, and lava, or Avith narrow streams of lava in 
