164 W. M. Hutchings — An Interesting Contact-Rock. 



subject use the terra " tbermo-metaraorpliism " more and more, and 

 almost abandon the name of " contact-metamorphism," This may 

 be right, and most of us have more and more assumed that it is so, 

 perhaps without quite stopping to consider how much of simple 

 assiimptio7i is involved. The evidence has appeared strong against 

 any theory that something passed from the igneous rock ' into the 

 rocks among which it was intruded, something besides mere heat, 

 and which was vitally concerned in the alterations which took 

 place. But still it may not be wise to regard the subject as being 

 disposed of. We have yet so many things to understand, and so 

 many strange facts to harmonise and explain, that it will be well 

 to leave this an open question for some time to come ; and for the 

 present it may be better to retain the use of the term " contact- 

 metamorphism " and not commit ourselves. 



Speaking for myself personally, I may say that continued study 

 of this fascinating subject has made me much more doubtful than. 

 I was, even a year or two ago, and more particularly since I have 

 had my attention specially fixed on the contact-eifects of basic rocks. 

 Some of the points to be considered are not quite easy to get over, 

 and are certainly not by any means settled. 



That fused material, actual igneous magma, passes over into and 

 is incorporated with the invaded rocks to any extent, is probably an 

 idea safely to be rejected, and is indeed probably held by very few 

 geologists. We may have areas of considerable and intricate 

 permeation, as for instance described by Mr. Barrow in the South- 

 Eastern Highlands (see Quart. Journ. Geol. Soc, vol. xlix. p. 331). 

 This, however, is a different matter and easily distinguished. 



But take the case of the intrusion of a granite into a mass 

 of slates. We know there was water in the slates, and we assume 

 that to the action of this water are largely due the changes which 

 are worked in the rocks under the influence of the heat of the 

 intrusion. We must admit a certain, and indeed considerable, amount 

 of freedom of movement of this heated water, and of the solutions 

 to which it gives rise, within the invaded rocks. But we know also 

 that the granite was charged with considerable water, or more 

 properly with solutions, when it was intruded, and it does not 

 appear that we have any reason to suppose that at the junction 

 of the two rocks there shall have existed a barrier such that none 

 of these solutions could pass. It is more reasonable to suppose that 

 at the junction, and for some considerable distance beyond it, the 

 temperature of the invaded rock, already more or less high owing 

 to the depth of cover, will soon be raised to something not too 

 far from that of the margin of the granite, and that solutions will be 

 able to pass for a time freely. It not only seems natural that this 

 should be so; it seems almost impossible to assume anything else 

 under the conditions we are bound to suppose. 



Now, as regards granite-intrusions, the chemical evidence is 

 usually held to show that no transfer takes place. But if we look 

 at this evidence we shall see that it is not, perhaps, so strong as we 

 have supposed. It mostly bears on the alteration of slates or allied 



