THE NATURAL SYSTEM OF VOLCANIC ROCKS. 



In reviewing the various attempts which have been made towards a classifica- 

 tion of eruptive rocks — that is, those crystalline rocks made up of silicates, which, 

 without showing themselves any traces either of stratified deposition or foliation, enter 

 into the structure of the surface of the globe in such a way as to be unconformable 

 with the stratification of the neighboring sedimentary rocks, and as a rule to abut 

 against them without any gradual passage — we are struck by the observation that, if 

 they are based on any principles at all, these are usually artificial, while none but un- 

 satisfactory results have been obtained when the application of natural principles has 

 been tried. This want of success is the more striking if we consider that it is peculiar 

 to petrology, and that the efforts made in the same direction with other branches of 

 descriptive natural sciences have been attended by extraordinary results. In zoology 

 and botany, the natural system has long since been considered as the ultimate object 

 of scientific research ; and since the time when its first outlines were discovered, the 

 progress of these scieuces has been admirable. Since then only have the developments 

 of their different branches cooperated harmoniously towards one common end : the 

 profoundest investigations into the anatomy of animals and plants, the study of their 

 geographical distribution in modern time, and of their gradual development in past 

 ages, have in their final results but been subservient to the establishment of a founda- 

 tion of the natural system, and the ingenious deductions made by Mr. Darwin on the 

 origin of species are but its philosophical interpretation. As regards mineralogy, clas- 

 sification was for a long time a simple enumeration of minerals, governed by cer- 

 tain artificial principles. A new era was inaugurated for this science by the progress 

 of chemistry, and its application to mineralogy, by Berzelius. It led to a more correct 

 estimate of those principles which had been formerly applied, and to the discovery of 

 the existence of an intimate connection between crystallographical form and chemical 

 composition. The combination of these two principles gave rise to the natural system 

 of minerals, which since their adoption has been constantly gaining in completeness. 



These are results which surpass in a surprising degree those obtained in regard 

 to the natural classification of eruptive rocks. Even the most recent and elaborate sys- 

 tematical arrangements, as those proposed by C. F. Xaumann, F. Senfft, B. v. Cotta, 

 and J. Roth, though marking a conspicuous progress, are based on almost purely arti- 

 ficial principles. In no other branch of the descriptive natural sciences, it is true, do 

 difficulties arise so great as those which present themselves in petrology. Prominent 



MEM. CAL. ACAD. SCI. VOL. I. F Jan. 1868. (43) 



