28(5 Mr. T. M. Reade on the Effects of Contraction 



pressed and in contact with the surrounding walls. When 

 the granite has by cooling solidified sufficiently to be un- 

 affected in this way, other agencies come into play. I have 

 shown * that variations of temperature of a small amount 

 extending over a large area will produce an intermittent 

 creep of the strata in one direction, so that the surrounding 

 rock is continually being forced up against the granite. 



That considerable variations of temperature are con- 

 stantly taking place in regions of intrusion is tolerably 

 certain, and that the heat of these intrusive masses is very 

 slowly imparted to the encircling rocks is shown by the 

 extremely slow cooling of some lava-streams that are only 

 protected from the atmosphere by a thin covering ; whereas 

 granite is often in enormous masses and deeply buried. 

 Some of these masses must have taken thousands of years to 

 cool. In addition to the lateral creep already spoken of, there 

 is another force tending to compression in the gravitation of 

 the surrounding and partially uplifted rock which naturally 

 tends to close up any shrinkage of the intrusive mass. 



It follows from this that granite-masses do not show 

 signs of shrinkage such as Forbes looked for, not because 

 the material does not shrink but because, from one reason 

 or another, it is kept throughout its history in a state of 

 compression. When the contraction of the under-mass takes 

 place on a large scale, then the granite may be sheared by 

 normal faulting, as I have described in chap. viii. ' Origin of 

 Mountain-Ranges,' hke any other rock-mass. 



Granite intrusion may take place without creating much 

 disturbance, but when it occurs in large masses, such as in 

 the core of Mont Blanc and in other Alpine regions, from 

 the amount of new matter forced in from below, it must 

 become a very potent factor in producing lateral pressure. 

 The fan-structure which we see in such regions is in my 

 opinion produced by the gradual spreading out of the 

 probably semi-plastic matter by gravitation ; indeed it seems 

 impossible that it can be due to any other cause. This idea 

 is not a new one by any means ; the celebrated architect 

 Viollet le Due has shown that expansion and intrusion of 

 the protogene in the core of Mont Blanc has caused the 

 folding back of the beds which it has forced apart f. The 

 effects of gravitation as a geological agent has lately been 



* Origin of Mountain-Ranges, p. 329. 



t Mont Blanc, by Eugene Viollet le Due ; translated by B. BucknaU, 

 1877, chap. i. 



