DUMONT ON THE CLASSIFICATION OF ROCKS. 25 



nized as a boracic compound. According to Frankenheim, the cry- 

 stals, attaining a size of from 1 to 1|^ inch [German], differ from 

 those of Brongniartin previously known in their angles, but slightly 

 however; the form also somewhat differs. Sometimes the crystals 

 appear perfect and transparent, sometimes white and laminated, the 

 fissures being occxipied by the above-mentioned substance. Spec. 

 grav. = 2-64 ; hardness=2-5-3*0. Its behaviour in the alembic and 

 before the blowpipe is like that of the Spanish Brongniartin. An ana- 

 lysis gave — Lime 19'6 



Soda 21-9 



Sulphuric acid 55*0 



Boracic acid 3*5 



100-0 



Formula : NA S + Ca S 

 The presence of borax is no doubt due to the admixture of the 

 mineral substance in which the crystals are imbedded. 



[T. R. J.] 



On the Classification of Rocks. By M. Dumont. 



[Acad. Roy. de Belgique, Bulletins, 1852, t. xix. no. 5. p. 18.] 



In this communication M. Dumont proposes a distribution of rocks 

 and mineral deposits generally into three classes, according to the 

 mode of their formation, and the use of the word Geyserian as a de- 

 signation for the third of these classes*. 



The chemical as well as the physical study of the crust of the 

 earth is now beginning to engage a portion of that attention which 

 for some years has been almost exclusively devoted to palaeontology ; 

 nor can it be doubted that inquiries which may hereafter enable the 

 geologist to explain both the physical and chemical condition of the 

 earth's crust, are necessary to a right understanding of the past 

 history of its successive changes. M. Dumont appears to feel this 

 when he suggests the threefold division of the rocks and strata of the 

 earth above-mentioned, and the adoption of a new designation for one 

 of them. He observes that the terms Neptimian and Plutonian 

 cannot embrace all the forms of mineral deposits. The term Nep- 

 tunian naturally comprises all stratified deposits which have been 

 formed under the action of external causes, and have therefore been 

 called by Humboldt exogenes. They have been produced generally 

 under the influence of water, exhibiting phsenomena of a mechanical, 

 chemical, or physical nature, and often containing the relics of organic 

 bodies. Such strata which are quartzose, slaty, clayey, calcareous, 

 dolomitic, or carbonaceous, and are either laminated, compact, sandy, 

 conglomeratic or organic, sometimes appear nearly in the condition of 

 their original deposit, and sometimes in a state of great alteration 

 consequent upon the action of internal causes subsequent to their 

 deposition, a change in consequence of which they have been desig- 



* [For remarks on a class of rocks termed JEolian, see Captain Nelson's Com- 

 munication on Coral Formations in this Number of the Journal. — Ed.] 



