BASALTIC FORMATIONS. 
115 
the Swedish word " trappa,^'' a stair, because many of 
them divide into regular forms resembling the steps 
of stairs. Though this distinction is not universal- 
ly appUcable to rocks of igneous origin, yet every 
person who has had opportunities of observation 
must have noticed that many rocks of this class do 
occur in tabular masses of unequal extent, so as to 
form a succession of terraces or steps on the sides 
of hills, as at the Palisades on the Hudson, and 
West Rock near New-Haven. Bakewell observes 
that whether the term, in its literal sense, is well 
chosen as a generic name, may be doubted ; but, 
taken metaphorically, it is extremely appropriate, 
as these rocks offer a series of gradations or steps, 
over which the geologist may safely travel in his 
speculations, from the lava of -^tna to the granite 
of the Alps. 
Mineral composition and texture. — The most com- 
mon varieties of volcanic rocks are, in regard to 
their composition, basalt^ greenstone^ syenitic green- 
stone^ clinkstone, clay stone, and trachyte ; while those 
founded chiefly on peculiarities of texture are por- 
phyry, amygdaloid, lava, scoricB, pumice, and tuff: all 
these are composed mainly of two minerals, /e/c?- 
spar and hornblende. 
We include augite under hornblende, because we 
think it proved that the same substance may assume 
the crystalline forms of hornblende or augite indif- 
ferently, according to the more or less rapid cool- 
ing of the melted mass. It is true that augite often 
has more lime and less alumine in it than horn- 
blende; but these elements vary; and it is now as- 
certained by the discoveries of Mitscherlich, of Ber- 
lin, that the ingredients of a given species of min- 
eral are not absolutely fixed as to their kind or qual- 
ity, but one ingredient may, within certain limits, 
be replaced by an equivalent portion of some analo- 
gous ingredient, without affecting the form of the 
crystal. This law has been called isomorphism^ from 
