CaL] FLUTOWKJ KOCKS. 7 



have been thrown out, and lavas have flowed over the land or bed of the 

 sea, at many different epochs, or have been injected into fissures; so that 

 the igneous as well as the aqueous rocks may be classed as a chronologi- 

 cal series of monuments, throwing light on a succession of events in the 

 history of the earth. 



Plutonic rocks (Granite, <fec). — We have now pointed out the exist- 

 ence of two distinct orders of mineral masses, the aqueous and the 

 volcanic : but if we examine a large portion of a continent, especially if 

 it contain within it a lofty mountain range, we rarely fail to discover 

 two other classes of rocks, very distinct from either of those above 

 alluded to, and which we can neither assimilate to deposits such as 

 are now accumulated in lakes or seas, nor to those generated by 

 ordinary volcanic action. The members of both these divisions of 

 rocks agree in being highly crystalline and destitute of organic remains. 

 The rocks of one division have been called plutonic, comprehending 

 all the granites and certain porphyries, which are nearly allied in 

 some of their characters to volcanic formations. The members of the 

 other class are stratified and often slaty, and have been called by 

 some the crystalline schists, in w T hich group are included gneiss, 

 micaceous-schist (or mica-slate), hornblende-schist, statuary marble, 

 the finer kinds of roofing slate, and other rocks afterwards to be 

 described. 



As it is admitted that nothing strictly analogous to these crystalline 

 productions can now be seen in the progress of formation on the earth's 

 surface, it will naturally be asked, on what data we can find a place for 

 them in a system of classification founded on the origin of rocks. I 

 cannot, in reply to this question, pretend to give the student, in a few 

 words, an intelligible account of the long chain of facts and reasonings 

 by which geologists have been led to infer the analogy of the rocks in 

 question to others now in progress at the surface. The result, however, 

 may be briefly stated. All the various kinds of granite, which consti- 

 tute the plutonic family, are supposed to be of igneous origin, but to 

 have been formed under great pressure, at a considerable depth in the 

 earth, or sometimes, perhaps, under a certain weight of incumbent 

 water. Like the lava of volcanoes, they have been melted, and have 

 afterwards cooled and crystallized, but with extreme slowness, and under 

 conditions very different from those of bodies cooling in the open air. 

 Hence they differ from the volcanic rocks, not only by their more crys- 

 talline texture, but also by the absence of tuffs and breccias, which are 

 the products of eruptions at the earth's surface, or beneath seas of 

 inconsiderable depth. They differ also by the absence of pores or cel- 

 lular cavities, to which the expansion of the entangled gases gives rise 

 in ordinary lava. 



Although granite has often pierced through other strata, it has rarely, 

 if ever, been observed to rest upon them, as if it had overflowed. But 

 as this is continually the case with the volcanic rocks, they have 

 been stvled, from this peculiarity, " overlying" by Dr. MacCulloch : 



