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 applicable 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 iEtna 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, syenilic green- 

 stone, clinkstone, clay stone, and trachyte ; while those 

 founded chiefly on peculiarities of texture are por- 

 phyry, amygdaloid, lava, sconce, pumice, and tuff: all 

 these are composed mainly of two minerals, feld- 

 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 



