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The petrographer who is examining a hand specimen is very much in 

 the position of a biologist who is dealing with a small fragment of some 

 unknown organism. Under such circumstances he (the biologist) does not 

 endeavour to give the fragment a distinct name, but to discover the nature 

 of the organism of which it formed a part, and the relation of the fragment 

 to the whole. Such individuality as rocks possess is dependent on their 

 relation to other rocks ; or, in other words, on their mode of occurrence in 

 the field. The rock specimen with which the petrographer deals in his 

 laboratory is, therefore, only a fragment ; and his object should be not merely 

 to describe the characters of the fragment, but also to determine its relation 

 to the larger mass of which it formed a part. 



Remembering then, that rocks are aggregates ; that different varieties 

 often shade into each other by the most insensible gradations ; and that 

 rock specimens possess in themselves no marked individuality, we conclude 

 that the primary object of the petrographer should be to describe rather 

 than to name the specimens which come under his notice ; and, further, that 

 in framing a terminology he should endeavour to make it as elastic as 

 possible, in consequence of the extremely protean nature of the objects with 

 which he deals. It cannot be too often remembered that language is merely 

 a means to an end, and that to import into the nomenclature of a science a 

 greater amount of rigidity than is warranted by the facts of nature, tends 

 to retard rather than to assist the growth of knowledge. 



Much of the difficulty which the petrographer experiences will be 

 removed if he distinctly recognises that it is no part of his duty to give 

 a definite name to every variety of rock which comes under his notice. 



As we must have a classification of some kind, for convenience of 

 description, it now remains for us to consider the principles available for 

 such a classification, so far as the igneous rocks are concerned. The 

 characters of igneous rocks group themselves under the following heads : 



(1) Chemical composition. 



(2) Mineralogical composition. 



(3) Texture. 



(4) Mode of occurrence in the field ; e.g., relations to surrounding 



rocks, structural features, &c. 



(5) Origin. 



(6) Geological age (distribution in time). 



(7) Locality (distribution in space). 



It is agreed on all hands that great importance should be attached to 

 the first three groups of characters. The principal difference amongst 

 petrographers has reference to the value which should be attached to 

 geological age. The dominant school of petrography upon the Continent 

 proposes to make geological age a primary factor in the classification of 

 igneous rocks to distinguish sharply between pre-tertiary and post- 

 cretaceous rocks and to use a different nomenclature for the rocks of the 

 two periods, even when they are absolutely identical in composition, texture 



