Origin of the CnjdaUine Rocks. 467 



it is diflScult to separate them. According to those who hold this 

 plutonic view the crystalline rocks represent the igneous crust of 

 the globe, and their frequent stratiform structure is due to agencies 

 in great part anterior to the production of sedimentary rocks. In 

 opposition to this view is that of the Neptunist, who, starting from 

 the fact that the elements of an aqueous sediment may through the 

 action of chemical and crystallogenic forces, pass into new combina- 

 tions and acquire a new structure, argues not only that all indigenous 

 crystalline rocks have had an aqueous origin, but that the exotic 

 masses themselves represent the last stages in this process of altera- 

 tion or metamorphosis of sedimentary beds. 



Further inquiry into the chemical and lithological composition of 

 the crystalline rocks, however, brings to light difSculties in the way 

 of both of these hypotheses. To begin with the Plutonist view, 

 volcanic rocks, both ancient and modern, are more or less nearly 

 related in composition to the gneisses and the stratified greenstones, 

 but we seek in vain among undoubted volcanic or igneous rocks for 

 the chemical representatives of the masses of serpentine, olivine, 

 steatite, chlorite, quartzite, magnetite, oligist, and limestone, which 

 appear in the primary formations, and have, all of them, by geologists 

 of the school in question, been regarded as of igneous or plutonic 

 origin. To account for the presence of such rocks among the more 

 or less felspathic aggregates — chiefly gneisses and greenstones, 

 which make up the greater portion of the crystalline formations — 

 three hypotheses have been imagined by Plutonists. According to 

 the first of these, the earth's interior is a reservoir, from which at 

 times have been ejected not only basic and acidic felspathic rocks, 

 but molten masses of olivine, iron-oxide, quartz, and limestone. 

 Other geologists of this school have sought to account for the presence 

 of some of these exceptional rocks by a process of so-called segrega- 

 tion, which would assimilate them to endogenous masses. The 

 chemical and geognostical difiiculties in the way of both of these 

 hypotheses have, however, led to their general rejection for the 

 third, which supposes these rocks to have been formed by a subse- 

 quent local alteration of portions of the ordinary plutonic rocks. 

 From acknowledged cases of alteration or replacement in mineral 

 species, which result in pseudomorphs, and from the more frequent 

 cases of envelopment and isomorphism which have been taken for 

 examples of pseudomorphism, it was argued that many species are 

 capable of being changed into others by the loss or addition of certain 

 elements, so that the resulting body often contains no portion of its 

 original constituents. Extending this view from single crystals to 

 rock masses, it was maintained that different portions of an igneous 

 or plutonic formation, whether basic or acidic, might be transformed 

 into serpentine, chlorite, or limestone. Those changes were supposed 

 to depend on the action of water, which, aided by heat, was regarded 

 as the efficient agent in the local alterations of plutonic rocks. At the 

 same time, the adjacent sedimentary strata were supposed to share 

 in these changes, thus giving rise to what have been called contact 

 formations. In their latest form these doctrines have been well set 



