DELESSE ORIGIN OF ROCKS. 5 



It is otherwise easy to comprehend why it is so ; for the chemical 

 and molecular actions which give rise to the mineral are exerted it 

 may be in the presence of heat, or it may be in the presence of water. 



But, by a fortunate circumstance, an identical mineral presents 

 great differences in its characters according to the rocks in which it 

 has been developed. 



It suffices, to appreciate these differences, to compare the vitreous 

 orthose of trachyte with the orthose of granite ; the augite of vol- 

 canic rocks with the pyroxene-diopside, or sahlite; hornblende of 

 basalt and of phonolite with the amphibolc-tremolite, or actinote ; 

 the peridote of lavas with batrachite. 



The decidedly special characters that each mineral takes, according 

 as it is formed in the presence of heat or in the presence of water, 

 will thus reveal its origin. 



§ 10. The order of solidification of the minercds which compose a 

 rock is not that of their fusibility, is then considered. 



§ 11. The characters of a rock depend on its chemical composition 

 and, its origin. — When one studies the eruptive rocks, it is easy to 

 discover that their chemical composition is simple, and moreover but 

 little varied. They contain generally silex, alumina, oxide of iron, 

 magnesia, lime, potash, soda, and water. Other substances are pre- 

 sent, but in a very slight proportion. 



The comparative examination of analyses of rocks teaches that, 

 when they have the same chemical composition, they may notwith- 

 standing present very different physical characters. 



Thus the composition is sometimes the same in trachyte and 

 granite, in basalt and trap, and in granite and eivrite. But if 

 trachyte encloses the minerals of granite, it is well distinguished from 

 it by its cellular structure and by the vitreous lustre of its felspar. 

 Well-marked differences exist equally between the other rocks which 

 I have cited ; and although their composition might be identical, no 

 geologist would ever confound them. 



It is necessary to go back to the very origin of these rocks to find 

 the explanation of their differences. At the same time that heat, 

 for example, accounts for the cellular structure and vitreous lustre of 

 trachyte ; water, pressure, and molecular actions give, on the other 

 hand, to other rocks the characters which distinguish them. 



The differences presented by rocks having the same elementary 

 composition are clue, then, to their origin. Consequently if the chemi- 

 cal composition of a rock exerts a great influence upon its characters, 

 the causes which have presided at its formation exercise apparently 

 a still greater. 



§ 12. An eruptive rock which contains ivater is not necessarily de- 

 composed. — When an eruptive rock is decomposed, it generally con- 

 tains water. Thus it is easy to seo that tbc granitic or trappcan 

 rocks pass first into a state of sand ; afterwards, proportionately as 

 the decomposition of their felspathic parts progresses, they take up 

 a greater quantity of water ; and lastly, they become cbanged either 

 into clay, more or less impure, or into kaohn. It thus is certain that 

 their quantity of water is augmented. If they contain carbonates, 



