W. M. Hntchings — An Interesting Contact-Rock. 169 



It is highly striking and curious to view with a low power the 

 uniform colourless field, and then to push in the analyser and see 

 these perfectly dark spots appear all over the slide. 



The darker band of slate, cut in the same sections, is also mainly 

 white mica and contains large amounts of the garnets, but also much 

 iron ore in dark grains, and abundant minute flakes of transparent 

 red hematite. There are here, also, spots of the same size and form, 

 in the same number and diffusion, but they are marked off in 

 ordinary light quite strongly, because they contain accumulations of 

 small grains of red iron ore. In polarized light they are still more 

 strongly marked, as in addition to the colour of the ore they have 

 the darkness due to isotropic matter. 



If one had these rocks shown to one, not knowing anything of 

 their origin, one would be almost certain to refer them to contact- 

 action. 



No doubt depth-temperature may be of the greatest importance in 

 influencing the degree to which rocks will be affected by contact- 

 action. A deeply-bedded rock with a high terapprature may be 

 naturally expected to be more easily and more intensely aifected by 

 an intrusion than one less deeply covered. Good geological evidence 

 exists in some cases to show that this has distinctly been so. 



Again, rocks that have been intensely folded and crushed may 

 have had their temperature greatly raised in the process, and may 

 be also in other ways, by the effects of minute cracking and fracture, 

 rendered more susceptible to the efii'ects of an intrusion taking place 

 immediately afterwards. • 



But the greatest depth-temperature, or the most intense dynamic 

 action, may be powerless to do more than to prepare and assist, 

 so far as concerns the special changes we have known as contact- 

 metamorphism ; and these changes really may depend upon the 

 chemical action of an intruded igneous mass. 



The whole of this interesting subject of the metamorphism of 

 rocks is as slippery and dangerous as it is fascinating, making ita 

 treatment the ground for a battle-field in which the war-cries 

 of "dynamic," "contact," and "thermal" are sometimes urged 

 with a force not equalled by the strength of evidence at command, 

 and in which the effects on combatants have certainly been occasion- 

 ally of a thermal nature. I only venture to timidly set my foot into 

 a part of one corner of this dangerous field because it has seemed 

 desirable to draw attention to one or two points of possible im- 

 portance to consider further, before we take what is yet only an 

 assumption for a proved case, and thus perhaps delay our arrival 

 at the truth ; an arrival which will in any event be probably long 

 postponed, in the matters we are now considering, owing to the 

 special diflSculties surrounding the investigation and to the scarcity 

 and doubtful nature of a good deal of the evidence we can hope 

 to collect. 



' Mr. Barrow has pointed out {op. cit.) how greatly intense folding of beds may- 

 increase their depth of cover, and so their temperature. He also alludes to some of 

 the evidence that greater depth, and its consequent higher initial temperature, can 

 be shown to have influenced the degree of contact-metamorphism. 



