166 W. M. Hutchings — An Interesting Contact- Roclc. 



agents " wliich have been so much written about, though their 

 nttture was left more or less vague and mysterious.' 



The question, indeed, may be asked as to why, if nothing but 

 thermal effects are concerned, we do not get rocks which have 

 undergone changes similar to those with which we are familiar as 

 "contact-action," but under the influence of depth-temperature alone? 

 We are acquainted with rocks of immense age, and which have been 

 covered up to enormous depths, corresponding to very high tempera- 

 tures, but which do not show any of the special characteristics of 

 contact-metamorphism ; — not one of the particular minerals which 

 we have learned to associate with it ; not even a trace of brown 

 mica, which we know is formed easily even at the comparatively 

 outer portions of contact-areas. And this is true of rocks such as 

 chloritic phyllites, which are specially sensitive in this respect. 



There are not wanting good reasons for thinking that there may 

 be an abrupt line of separation between the effects of " contact- 

 action " and any other process with which we are acquainted,^ and 

 that a list of minerals, for instance, could be stated, whose develop- 

 ment in sedimentary rocks, so far as our knowledge goes, takes 

 place only as an effect of contact-action. 



If that should be so, we should not be justified in speaking of 

 this action as simply thermal, unless we can find evidence that 

 elevation of temperature alone has ever produced, or can produce, 

 these minerals. 



Round every mass of intruded igneous rock which we are able to 

 stud}^ we find certain definite results produced, — physical changes 

 effected, and new minerals developed. We find the same things 

 over and over again, and learn to look upon them as quite assured 

 characteristics of the rocks round such intrusions. 



If we meet with the same characteristic minerals in all respects, 

 but where no intrusion of igneous rock can be discovered, we are not 



1 Of course, it does not follow that nothing but alkalies would be transferred, 

 though alkaline compounds, mainly silicates, would be far away the principal 

 material. We may well suppose that other matters would also be present, and that 

 among them would be small proportions of fluorine-compounds, which have long 

 been known to exercise such a very powerful influence in synthetic experiments on 

 the formation of minerals, and which may, as has, indeed, been frequently supposed 

 and stated about them, exercise a similar power in starting and maintaining the 

 metamorphic processes in the rocks. 



^ If this be so, and a rock cannot be metamorphosed simply by heat and the action 

 of its own confined water, we cannot look for any success in experiments on samples 

 of rock heated in platinum, or other sealed metallic containers, to retain the water. 

 This may be the reason why so little is recorded in this direction, and even a portion 

 of what is recorded is not satisfactory evidence. There may have been many failures 

 not recorded at all, thoxxgh it is almost as valuable, in many cases, to record failures 

 as successes. I will set the example of recording failures by stating that recently 

 I have heated a flre-clay, rich in alkali, in a moist condition, to a low red heat for 

 one mouth in a wrought-iron bomb. All the water was retained perfectly, but the 

 clay was not affected in any way, except that its combined water was 1 per cent, less 

 after the experiment, showing that some of its hydrous minerals had been dehydrated 

 by the red heat. This clay would have been very susceptible to metamorphism if it 

 had been invaded by a granite or a diabase. Subsequent experiments at a strong 

 red heat failed entirely, as my bombs gave way uuder the pressure of the contained 

 water- vapour, the iron getting too soft at the higher temperature. 



