Ch. XL] THE PRIMARY ROCKS. 219 



stratified, and produced from aqueous sediments, as gneiss, 

 mica- schist, argillaceous schist, and some others ; the second, 

 or unstratified, which is supposed to have been formed by 

 igneous action at great depths, whereas the volcanic rocks, 

 although they also have arisen up from below, have cooled 

 from a melted state upon or near to the surface. The un- 

 stratified primary, very commonly called Plutonic, often occur 

 in large amorphous masses, from which small veins and dykes 

 are sent off, which traverse the primary stratified rocks pre- 

 cisely in the same manner, in which lava is seen, in some 

 places, to penetrate the secondary strata. Thus we find one 

 set of granite- veins intersecting another ; and granitiform 

 porphyries intruding themselves into granite, like the volcanic 

 dikes of Etna and Vesuvius, where they cut and shift each 

 other, or pass through alternating beds of lava and tuff. 



There is, however, a connecting link between the volcanic 

 and Plutonic rocks, which makes this analogy much greater. 

 The class of unstratified rocks, called trap, are compounds 

 chiefly of felspar with hornblende or augite, which are asso- 

 ciated with the fossiliferous rocks intersecting, overlying, 

 and penetrating them as dikes, veins, beds, and amorphous 

 masses. At first, geologists were divided in opinion, as to 

 whether trap rocks were of igneous or aqueous origin ; but 

 now they are admitted to have been once in a state of fusion ; 

 it having been ascertained that the early comparison of trap 

 to lava was imperfect, because a set of rocks, formed almost 

 entirely under water, was contrasted with another which had 

 cooled in the open air. Dr. Macculloch observes, " that it 

 is a mere dispute about terms, to refuse to the ancient 

 eruptions of trap the name of submarine volcanos ; for they 

 are such in every essential point, although they no longer 

 eject fire and smoke." If a division, then, be attempted 

 between the trappean and volcanic rocks, it must be made 

 between different parts of the same volcano, nay, even the 

 same rock, which would be called trap where it fills a fissure, 

 and has assumed a crystalline form on slow cooling, must be 



