GNEISS. 127 



CHARACTERS OF THE IGNEOUS PRODUCTS. 

 In drawing our description of igneous rocks to a close, we 

 would suggest that the student should commit to memory the 

 following resume :- 



1. The granites and their associate rocks, have "been formed 

 under conditions of subterranean character, and have melted, 

 and solidified under the pressure of the earth's crust. 



2. The trap-rocks, the porphyries, clinkstones, and ba- 

 salts, are to be regarded as submarine, and as having been 

 melted, and become solid beneath the depth and pressure 

 of the sea. 



3. The volcanic rocks are sub-aerial, and have melted, 

 and solidified near the surface, with access of the atmosphere. 



Finally, these productions are so similar, that it is by 

 no means unreasonable to suppose that plutonic, trappean, 

 and volcanic rocks, may all be simultaneously forming at 

 the present moment, and that the same volcano which ejects 

 lava and scoriae from its summit, may, in its internal abysses, 

 at enormous depths and under vast pressure, be elaborating 

 granite; while, from a neighbouring vent, beneath the 

 adjacent seas, it is spreading its waves and terraces of trap 

 over the floor of the ocean. 



METAMORPHIC ROCKS. Prom the examination of the 

 plutonic and the volcanic rocks, we pass to the consideration 

 of these deposits, which, according to the views now gene- 

 rally entertained, were originally deposited as sediments 

 from water, but have been subsequently altered by subter- 

 ranean heat. 



GNEISS. Gneiss may be termed stratified, or slaty granite. 

 Composed of the same elements, mica, (or hornblende,) quartz, 

 and feldspar, it differs from that rock, which presents no 

 appearance of stratification, in this distinctive character, 

 that some of its component minerals, usually the mica and 

 hornblende, are arranged in layers, which give a slaty 

 aspect to the mass. The stratification is irregular and con- 

 torted, and the dimensions of the strata are variable. When 

 the gneiss is associated with granite, it approaches to the 

 character of that substance ; and when the two come in con- 

 tact, it is scarcely possible to distinguish between them. It 

 is connected with the other primary strata, in the following 

 modes : By the disappearance of feldspar, it passes into 



