652 A TREATISE ON METAMORPHISM. 



hundreds of feet in thickness and many miles in extent, in the Animikie 

 series of the north shore of Lake Superior." 



(f) The higher the temperature of the intrusive the greater the 

 amount and extent of the contact effect. The temperatures of lavas as 

 they reach the sui'face vary greatly, ranging from 700 to 1,100° C. 6 

 Other things being equal, it is evident that those magmas which have the 

 higher temperatures are more effective in metamorphism, for two reasons. 

 In the first place, they furnish a larger amount of heat to the surrounding 

 rocks and solutions; and in the second place, they have greater mobility 

 and therefore are more likely to extend far and enter minute openings. 



(g) Other things being equal the greater the depth of the intrusion 

 the greater the contact effect. One of the chief reasons for this is that 

 rocks are very poor conductors of heat, and therefore the direct and 

 indirect effect upon the intruded rocks continues for a longer time with a 

 gradually broadening zone of action. Indeed, it is highly probable that 

 the larger masses of the intrusive rocks in the middle and lower parts of 

 the belt of cementation, of Tertiary age, have not yet cooled. The solu- 

 tions of very greatly increased metamorphic efficiency on account of this 

 heat have been at work for millions of years. 



Finally, the pressure is much greater where the intrusives are deep 

 seated, and this promotes metamorphism. This matter of the increase of 

 the metamorphic power of intrusives with depth can not be too strongly 

 emphasized. Indeed, in cases of widely extended and profound exomorphic 

 effect the intrusives are almost invariably deep seated. As the depth of the 

 intrusives increases, we gradually pass from the belt of cementation, which 

 we are now considering, to the zone of anamorphism, to be considered in 

 the following chapter. It will there be seen (pp. 711-736) that in that zone, 

 where the intrusives are of still greater depths than in the belt of cemen- 

 tation, the contact effects are of the most profound character. But it can 

 not be too strongly insisted upon that these effects are not mainly due to 

 the direct heating effects of the intrusive rock, but to the indirect effect 

 upon the solutions in heating them and furnishing materials to them. 

 I repeat, the solutions are the important direct agents which produce 

 far-reaching and profound metamorphism in the belt of cementation. 



-'Lawson, A. C, The laccolitic sills of the northwest coast of Lake Superior: Bull. Geol. and Nat. 

 Hist. Surv., Minnesota, No. 8, 1S93, pt. 2, pp. 24-48. 



&Geikie, A., Textbook of geology: Macmillan & Co., London, 1893, pp. 225-226. 



