Descriptive Arrangement of Volcanic Rocks. 39 



2. The form of the fragments must also be noticed. This 

 is either, 



a. angular, ^. water-worn, 7. rolled. 



3. The fragments should be referred, if possible, to some 

 mineral species of iithoidal rocks, and their varieties, if any, 

 taken notice of; as well as the occurrence of isolated crys- 

 tals, rare minerals, shells, wood, &c. 



4. The solidity of the conglomerate rock ; which may be, 

 A. incoherent, or earthy, B. indurated. 



5. The divisionary structure, which is occasionally met 

 with in conglomerate as well as in Iithoidal rocks, and is sub- 

 ject to the same varieties of form. 



The volcanic rocks, both Iithoidal and conglomerate, are 

 sometimes found in an altered state, from having been expo- 

 sed to the decomposing influence either, 



1. Of proximate emanations of aqueous vapours charged 

 with sulphuric and muriatic acids ; or, 



2. Of the ordinary atmospheric agents. 



In the first case, the alumine and potass of the felspar and 

 augite are taken up by the sulphuric acid, and deposited by 

 the agency of water, as sulphat of alumine (alumstone,) in 

 the cavities aud fissures of the rock, and in neighbouring hol- 

 lows ; leaving the remainder of the rock, composed almost 

 solely of silex, in a carious state, but often filled up with other 

 infiltrated matters as well as alum, and stained with ferrugi- 

 nous oxides, from the union of its iron with the oxygen of 

 the acids. In the second case, the decomposition of the au- 

 gite and felspar, sometimes of one, at others of both, produ- 

 ces a variety of argillaceous earths or boles, giving to the 

 rock, which is then often called wacke, a more or less argil- 

 laceous aspect, proportionate to the degree of decomposi- 

 tion, and sufficient to render it occasionally difficult to recog- 

 nise its original mineral composition. These boles are some- 

 times conveyed by aqueous infiltrations into the cellular and 

 other cavities of the rock, giving occasion to the amygdaloi- 

 dal composition. 



The object proposed in the foregoing remarks is to endea- 

 vour to establish a fixed nomenclature for the principal cha- 

 racteristics of the volcanic rocks, so as to enable any obser- 

 ver to define or describe all their varieties accurately and 

 distinctly, for the ulterior purposes of geology. Names may 

 be subsequently given by geologists to any of these varie- 

 ties, for the sake of avoiding a redundancy of words, or not. 



