STRATIFICATION IGNEOUS ROCKS. 



fusion within the terrestrial shell upwards to the surface, are very 

 numerous. 



They have, however, been reduced to the five following classes 

 proceeding upwards. 



1 The igneous rocks. 



2 The transition or metamorphic rocks. 



3 The secondary rocks. 



4 The tertiary rocks. 



5 The diluvial and alluvial layers of matter upon which the 

 superficial soil is spread. 



We shall first briefly notice these five great layers, each of 

 which, however, as will hereafter appear, consists of numerous 

 subordinate courses or strata. 



19. The lowest or fundamental layer, called igneous rocks, 

 consists exclusively of agglomerations of mineral masses in a 

 state of crystallisation. This is the condition which matter 

 would necessarily assume, and which it could only acquire by 

 having been gradually cooled and solidified, after being brought 

 to a state of fusion by a great elevation of temperature. Under 

 such circumstances its chemical constituents would group them- 

 selves according to their mutual affinities, and would assume the 

 various crystallised forms proper to them. After cooling and 

 solidifying, the materials would present the appearance of an 

 agglomeration of crystals thrown arbitrarily together and without 

 regularity or order. Now this being exactly the appearance pre- 

 sented by the rocks which form the foundation of the crust of 

 the globe, it is inferred that they were originally in a state of 

 igneous fusion, and that by the gradual loss of heat by radiation, 

 they were superficially cooled and solidified. The parts of the 

 primitive rocks which have been brought under the observation 

 of geologists are considered as forming the external parts of this 

 solid layer. 



These rocks are from circumstances here explained often 

 denominated PLUTONIC or IGNEOUS HOCKS, or BOCKS OF IGNEOUS 

 ORIGIN. 



20. Those primitive layers, which may be regarded as the 

 original materials of which the entire crust of the globe is formed, 

 consist chiefly of that rock familiar to all observers of mountainous 

 countries called GRANITE, the most imperishable of all stones, and 

 therefore the most precious for the purposes of construction. This 

 granite is mixed in the fundamental layer in smaller proportions 

 with the minerals called AMPHIBOLE, PYROXENE, and PERIDOTE. 



21. Granite is an agglomeration of the crystals of three 

 minerals, called FELDSPAR, MICA, and QUARTZ. Feldspar is the 

 soft grey part of the granite, which is easily scratched. The 



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