

METHODS OF PETROGRAPHICAL RESEARCH. 



IF we were required to fix an epoch from which modern 

 Petrology might date its progress, we should perhaps 

 with most justice select the year 1815, in which Louis 

 Cordier produced his Memoire sur les substances miner ales 

 dites ''en masse" qui entrent dans la composition des roc lies 

 volcaniques de tons les 'ages. The title itself is a landmark 

 in the history of geology. The so-called " massive felspar " 

 and other substances constituting the ground-mass of many 

 lavas were, eighty years ago, generally regarded as homo- 

 geneous minerals, and the conception of "volcanic rocks of 

 all ages " was then far from being the common-place which 

 it is at the present day. The French geologist demon- 

 strated conclusively that the compact ground-mass of very 

 many porphyritic rocks is in reality only a very fine-textured 

 aggregate of two or more minerals, and that these minerals 

 are identical with those already familiar as porphyritic ele- 

 ments in the same rocks. A discovery of such moment so 

 lucidly set forth is not easily paralleled. 



Scarcely less interesting are the methods — in great 

 measure original — by which Cordier was enabled to arrive 

 at such results at a time lono- anterior to the introduction of 

 thin slices of rocks. The microscope was for the first time 

 systematically employed in petrological research, though 

 only with low powers, for, working upon fragments of ir- 

 regular form obtained by crushing, Cordier was compelled 

 to sacrifice amplification to definition. The form, lustre, 

 colour, transparency, etc., of the mineral fragments were 

 thus studied, while other characteristics were found in the 

 brittleness or toughness of a mineral under the pestle, the 

 "feel' 1 of its powder, etc. In some cases the process of 

 " mechanical analysis" thus devised was aided by levigation 

 in water, and by the use of a bar-magnet to separate 

 different minerals in the pounded rock. The blow-pipe, 

 used by De Saussure thirty years earlier, was revived and 

 its application perfected, so as to determine with consider- 



