102 



PROP. T. G. BONNET ON A CONTACT-STRTJCTURE 



throw light upon some general questions in the history of igneous 

 rocks belonging to the more acid division. 



The consolidation of a rock may be regarded as a function of 

 three variables, more or less independent, these being heat, water, 

 pressure. The usual effect of a falling temperature is probably 

 greatly modified by the second and third ; so that, in nature, the 

 order of consolidation of minerals from a magma may vary much 



Fig. 1. — Structure of '* syenite^' close to junction, from Bradgate Parle, 

 The larger grains are quartz and felspar ( x about 30). 



from that of an anhydrous mass, otherwise of like composition, and, 

 in the process of cooling, variation in the amount of pressure or 

 water may produce apparently anomalous or contradictory results *. 



* Many of the remarks made in this paper are obviously not new. But, as I 

 have stated nothing which I have not observed for myself, and have had the sub- 

 ject constantly present to my mind since 1877 (see my paper ' On certain Rock- 

 Structures as illustrated by Pitchstones and Felsites in Arran,' Geol. Mag. 

 dec. ii. vol. iv., and my Presidential Address to the Society in 1885), I have not 

 attempted to stud these pages with references to the works of others, or to devote 

 hours of search through books in endeavouring to ascertain whether a particular 

 idea was published by myself before I could have had it suggested by the 

 •writings of anotlier. I have not read any paper which has treated the subject 



