10 DEVELOPMENT OF PETROLOGY. 



giavitv and highly endothermic character in their tornia- 

 tion under pressure, as being more characteristic of tlie 

 '■ infra-phitonic zone " than garnet. 



Within the last few months • Igneous Rocks," Vol. 11., 

 by J. P. Iddings, has been pubUshed. It is interesting 

 to note that the (classification of rock-groups is based 

 mainly on the old ' qualitative " system and not on the 

 "' American Classification "" of which Iddings is one of the 

 fonnderi-i. 



CONCLl'SION. 



An attempt has been made to deal with the various 

 advances made in ])etrologv during the last thirteen years. 

 Owing to the great number of publications bearing on this 

 question, and the difficulty of obtaining them here, it is 

 felt that this review cannot be other than incomplete : 

 but it is hoped that at least the main developinents have 

 been chronicled. With regard to the classifications- of 

 igneous rocks in vogue at present, those based upon (a) 

 mineral (or chemical) composition and (b) texture or 

 geologic mode of occurrence are perhaps the most satis - 

 factorv. It is hoped that as our knowledge becomes more 

 complete classifications may be based on the principles 

 of eutectics and the methods of genesis of igneous -types. 



The main objections to the existing American Classifi- 

 cation are that it can not be used without a fairly intimate 

 knowledge of the chemical composition of the rock, and it 

 usually replaces the actual mineral composition by an im- 

 aginary mineral composition (the norm). On the other 

 hand it has proved of very great service in revealing 

 chemical characteristics and relationships. 



There has been a great deal of controversy on the 

 ■■ Relations between tectonic and petrographical facies,'^ 

 as Marker terms it. Marker concludes that " a« regards 

 the younger igneous rocks, the main alkahne and calcic 

 regions correspond to the areas characterised by the Atlantic 

 and Pacific types of the coastUne respectively," and holds 

 that the alkaline rocks are typically associated with sub- 

 sidence due to radial contraction of the globe, and the 

 calcic rocks with folding due to lateral compression. 

 Cross and others, however, hold that whether the relation- 

 ship involved, is true or not, it is not responsible for the 



