1870.] MACFARLANE — ON CRYSTALLINE ROCKS. 47 



from which it commands a good view of the immediate neigh- 

 bourhood, and suddenly drops upon any unfortunate object in the 

 shape of food that may happen to pass within a convenient 

 distance. 



(To he continued.^ 



ON THE ORIGIN AND CLASSIFICATION OF 

 ORIGINAL OR CRYSTALLINE ROCKS. 



By Thomas Macfarlane. 



I . — I NTRODUCTION. 



"All attempts to separate sharply from each other the various 

 " rocks or mineral aggregates of which the earth's crust is com- 

 " posed, and to arrange them systematically, have failed." "We 

 "' cannot consider the rocks as species, nor arrange them in a 

 " system corresponding to their nature, nor even, in describing 

 " them, treat them all in the same manner." * 



So wrote Bernhard Von Cotta in 1862. On reading such 

 sentences we are tempted to ask : Are species always sharply 

 defined in other sciences ? Are all systems perfect or natural ? 

 Why should lithology be an exception to other sciences, and its 

 students be deprived of the advantages of a systematic arrange- 

 ment of the objects to be studied ? A "natural" system is not 

 demanded, even were such a thing possible, in this or any other 

 science. The more rigid any method of classification, and the 

 more marked and unbeading its divisional lines are made, the 

 more unnatural it becomes. 



It is exceedingly gratifying to find that, undeterred by the 

 difiiculties of rock classification, such lithologists as Von Hoch- 

 stetter, Kjerulf and Zirkel, have been found willing to attempt it. 

 Their labours, and those of other workers in the same field, have 

 shed a flood of light upon a previously obscure and uninteresting 

 subject. Although a perfect system will, perhaps, never be 

 attained, still each attempt at properly arranging our knowledge of 

 the subject has its value. Chemical analysis and microscopical 



* Cotta; Die Geisteiuslehre, pp. 1, 4. 



