SENFT ON ROCKS. 



The three following sections treat of 



I. The formation of rocks from crystalline minerals : (1) as regards 

 the distinction between simple and compound crystalline rocks ; 

 (2) as to the structure or texture of rocks, whether granular, 

 slaty, foliaceous, fibrous, compact, porphyritic, sphaeroidal, amyg- 

 daloidal, or vesicular. 



II. Rock-fragments as materials for the formation of rocks ; whether 

 round or angular, large or small, cemented together by clay, marl, 

 limestone, silex, &c. ; these are termed clastic rocks ; their struc- 

 ture is either psephitic, when the fragments are at least the size of 

 hazel-nuts, as breccias and conglomerates ; psammitic or sand- 

 stones ; pelitic or mud- and clay-deposits. 



III. Organic remains as materials for the formation of rocks, either 

 alone or mixed with or imbedded in other rocks ; as plants, corals, 

 shells, and infusorial remains. 



In the section on the systematic classification of rocks, the author 

 states the difficulties of making a good petrographic system of rocks, 

 founded on their structure, colour, or external characters': (1) as the 

 structure of rocks is in many cases so uniform that the ir dividual 

 component parts cannot be distinguished even by the aid of a mag- 

 nifying glass, as in melaphyre, aphanite, phonolite 3 basalt, &c. ; 

 (2) the mass of rock may vary in its range, being simple in one lo- 

 lity, and acquiring additional constituents in another, thereby 

 approaching the character of the mixed rocks, as pifchstone, pitch- 

 stone-porphyry, granular limestone, and calcareous mica-schist ; or 

 again, in one locality a certain mineral may predominate, which is 

 absent, or sparingly distributed, in the same rock in another locality; 

 or, further, to the mineral constituents of a rock may be added a 

 new substance, or one of the original elements may be replaced by 

 another mineral in more or less abundance, thus producing, or pass- 

 ing into, another variety of rock ; (3) the structure of the same rock 

 may vary, presenting either a granular, slaty, compact, porphyritic, 

 amygdaloidal, or even slaggy structure, as basalt, &c. From the 

 above reasons the author considers that the chief divisions of rocks 

 should not be founded on their texture, as one and the same rock 

 might otherwise be placed in different divisions, — or on external 

 appearance or colour, as rocks of very different composition might 

 be arranged together, while others, closely allied, would be sepa- 

 rated. 



In consequence of the difficulties arising from other principles of 

 classification, Dr. Senft proposes an arrangement based on mineral 

 and chemical composition; as the analysis of a rock affords the 

 clearest indication of its constituents : and hence he has adopted, so 

 far at least as leading to the determination of the mineral consti- 

 tuents of a group of rocks, the arrangement of which the following 

 analyses of his elaborate Tables present a general synoptical view, 



b2 



