2 6o SCIENCE PROGRESS 



ment as to the characters which should distinguish the different 

 classes and the nomenclature which should be employed, that 

 those who wish to keep in touch with the world's petrological 

 literature must make themselves acquainted with some half- 

 dozen systems of classification differing from one another in 

 many important points, and none of them entirely satisfactory. 1 



In almost all cases the main rock types are now defined by 

 the conjunction of certain mineral and structural characters, 

 though by some authorities considerations of structure are still 

 subordinated to stratigraphical relations. But while in this 

 country most petrologists take the mineral composition as the 

 basis of the larger divisions, others give this place to the 

 structural or stratigraphical characters. 



Chemical composition, on the other hand, has up to the 

 present played a comparatively small part in any system of 

 classification, in spite of the fact that in the early days of 

 geology Elie de Beaumont divided igneous rocks into acid, 

 containing more than 65 per cent, of silica ; intermediate, with 

 from 55 to 65 per cent., and basic, with 40 to 55 per cent. ; and 

 that a similar division has been made by many subsequent 

 writers, though somewhat different percentages may have been 

 substituted, or two groups have taken the place of three. 



The importance of a knowledge of the chemical composition 

 has long been recognised, and is now appreciated more than 

 ever ; but it is still the exception for chemical analysis to be 

 resorted to. The complete analysis of an igneous rock is a 

 long and complex operation, and few geologists possess the 

 technical skill and laboratory appliances to carry it out them- 

 selves. Professional analysts, too, however expert they may 

 be in other directions, have rarely the special training in rock 

 analysis which is necessary if satisfactory results are to be 

 obtained. Workers have therefore, as a rule, contented them- 

 selves with the microscopical examination, and as a result the 

 prevailing systems of classification are based on mineralogical 

 and structural characters, and only indirectly and approximately 

 express the chemical composition. 



While, however, the investigation of the chemical composition 



1 A full, though by no means exhaustive, account of the different attempts to 

 classify rocks, from the time of Linnaeus to the present day, will be found in the 

 form of an introductory review to the Quantitative Classification of Igneous Rocks 

 cited above. 



