Notes on the Igneous Rocks. 463 



Notes to Accompany a Tabulation of the 

 Igneous Rocks based on the System of 

 Prof. H. Eosenbusch. 



By Frank D. Adams, Lecturer in Geology, McGill University. 



Of all the plans proposed from time to time by various 

 authors for the classification of the Igneous Rocks, that by 

 Prof. Eosenbuscb of Heidelberg is the one which has met 

 with the greatest favour, and is now adopted by almost all 

 petrographers throughout the world. 



The classification proposed in the first edition of this 

 author's " Mikroskopische Physiographie der Massigen 

 Gesteine," published in 187*7, was altered in some essential 

 particulars in his becond edition of the work published in 

 1887, while the great advances in petrographical knowledge 

 since that date have led to the adoption of still further 

 modifications in the classification adopted for the unique 

 collection of rocks which he has brought together in the 

 museum of the Geological Institute of the University of 

 Heidleberg. 



Having drawn up a table incorporating the latest results 

 in this field in a condensed form, for the use of my students 

 at McGill College, I have ventured, at the request of a num- 

 ber of American petrographers whom I consulted while 

 constructing it, to publish it with a few words of explana- 

 tion, in the " Canadian Record of Science," that it might be 

 available for the use of students elsewhere. It is based 

 upon and in a general way resembles a table published bj T 

 Prof. Rosenbusch in 1882, in the "Neues Jahrbuch fur 

 Mineralogie, &c." 



There has recently been a tendency among petrographers 

 to consider rocks rather from a chemical than from a 

 mineralogical standpoint, as geological units having a cer- 

 tain chemical composition rather than as aggregates of 

 certain mineral species. This is in part owing to the fact 

 that magmas of diverse composition may crystallize out in 

 very similar mineral aggregates, thus for instance, a mod- 



