M. Dumont on the Classification of Rocks. 27 o 



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 phenomena of a me- 

 chanical, chemical, or physical nature, and often containing 

 the relics of organic bodies. Such strata, which are quart- 

 zose, 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 designated Metamorphic. The term Plutonian 

 comprises those rocks which have been produced by igneous 

 action from internal causes, and have been therefore called 

 by Humboldt endogenes. Such rocks are crystalline, and 

 sometimes cellular, are feldspathic, and appear either in 

 masses or have been erupted, like lavas, in streams. 



By the term " Geyserian" M. Dumont proposes to desig- 

 nate those rocks which, though, like the Plutonian, they have 

 been produced by causes acting from within, have not, like 

 them, been fused by heat, but have been formed by either 

 aqueous or gaseous emanations. The Plutonian, in fact, 

 have been formed like lavas, the Geyserian like sublimed 

 sulphur. Geyserian rocks are metalliferous, rarely feld- 

 spathic, are confusedly crystalline, concretionary, or cellular, 

 and exhibit a very different aspect to that of the Plutonian. 

 On the other hand, though sometimes conglomeratic or com- 

 posed of transported materials, and formed under the in- 

 fluence of water, they are distinguished from the Neptunian 

 by their want of stratification, by the metallic and mineral 

 substances they contain, by the absence of organic remains, 

 by a crystalline or concretionary structure, and especially by 

 their mode of formation. 



Such are the views of M. Dumont ; and although, as he 

 states, it may be sometimes difficult to draw the line of limi- 

 tation between rocks of these various modes of formation, 

 and the Geyserian may appear involved in, and subsidiary, 

 sometimes to the Plutonian, sometimes to the Neptunian, it 



VOL. LV. NO. CX. — OCTOBER 1853. S 



