292 SCIENCE PROGRESS. 



mineral constitution of the rocks produced and the com- 

 plexity of some of them are very striking. Calcareous beds 

 which seem to differ but little may give rise by meta- 

 morphism to quite different mineral aggregates. Presumably 

 slight differences in the chemical composition of the rocks 

 affected may determine wide diversity in the recombinations 

 set up in them, thus emphasising the prime importance of 

 the bulk composition as affecting the results of the meta- 

 morphism. 



It is only in recent years that any detailed study has 

 been made of thermal metamorphism in rocks of igneous 

 origin. The results promise to shed considerable light 

 upon the problems offered by large areas of crystalline 

 schists. It may be laid down as a broad rule that fresh 

 igneous rocks are much less susceptible to changes by 

 thermal metamorphism than are most ordinary sediments. 

 This is a simple consequence of the fundamental principle 

 of these mineralogical transformations, viz., the instability 

 of chemical compounds under changed physical conditions. 

 Substances such as the common weathering products, 

 formed at ordinary atmospheric temperatures, become un- 

 stable when subjected to much higher temperatures ; but 

 minerals which have originated from igneous fusion are, as 

 a rule, much less liable to change when heated. In both 

 categories there are a few exceptions, e.g., quartz under the 

 former head and the pyroxenes under the latter, but the 

 rule is of general application. 



Conversely, combinations entered into at high tempera- 

 tures, as in rocks formed from fusion, cease to be stable at 

 ordinary low temperatures ; and most igneous rocks which 

 are not geologically very young have suffered more or less 

 change from this cause. The transformations induced in 

 them are further connected with the access of meteoric 

 water, carbonic acid, oxygen, etc., as is evident from the 

 nature of the common "weathering" products. Igneous 

 rocks so altered are quite as readily affected by thermal 

 metamorphism as argillaceous or other sediments, the first 

 signs of change appearing always in the decomposition 

 products of the rocks. 



