W. M. Hiitchings — An Interesting Contact-Rock. 167 



taking a very risky step if we look upon it as probable that there is 

 au intrusion to account for what we see, but that it is hidden from 

 us. An exposure of plutonic rock is only an accident of denudation 

 after all. It seems a more risky step for us to assume that because 

 we do not here see the intruded rock, we can ascribe all the well- 

 known characteristics to some other cause, or even to no definite 

 cause at all. Dynamic metamorphism, for instance, has probably in 

 some cases obtained the credit for many things which possibly may 

 yet be shown not to belong to it at all, simply because dynamic 

 action has taken place in regions in which great contact-action has 

 also occurred, though the proof of the igneous intrusions is hidden, 

 while the dynamic evidence is clear. Some important recent work 

 has been in the direction of thus proving certain districts to be 

 contact-areas, in which the observed phenomena of the rocks had 

 been attributed to other causes. 



Great dynamic action and igneous intrusions are apt to go 

 together, as we know ; but whereas we find certain minerals in all 

 contact-areas, we know of plenty of areas of intense dynamic action, 

 where they do not occur ; not any trace of any of them. 



Great as are the demonstrable effects of dynamic metamorphism in 

 altering structures, in developing certain new minerals, and ia 

 regenerating others which are in a decayed condition, there are yefc 

 many minerals which we may decline to accept as proved to be 

 within its powers to produce, even when we see them in regions of 

 intense dynamic action. A great aim of future petrological study 

 will be the endeavour to establish something reliable on these points. 



It may be, and there would be reasons to be advanced for the 

 belief, that dynamic action ever so intense can ouly, so far as the 

 production of new minerals in a rock is concerned, do the same 

 things, on a much intensified scale and in less time, as can be done 

 by simple " metasomatic " action. White mica, for instance, is 

 unquestionably in the power of dynamic action to develop on a great 

 scale in felspai'-bearing rocks. But white mica is also produced 

 copiously during the simple decay of felspar. Garnet, again, may 

 be cited as a mineral produced by dynamic action and also, if not so 

 strikingly, by decay of other minerals ; and additional cases might 

 also be adduced. Some of them, like the two above, are also well- 

 known contact minerals, and so are common to all three causes. 



But andalusite, sillimanite, and cyanite, for instance, may be here 

 named as among the list above suggested as not yet proved to be 

 capable of formation in sedimentaries except by contact-action. 

 We may even still pause, and ask for proof that dynamic or regional 

 metamorphism can produce brown mica, that most constant and 

 copious accompaniment of contact-action; and the same position of 

 enquiry may also be taken as regards new felspar, that is, felspar 

 produced in rocks in which it did not exist before the meta- 

 morphism, as in many slates and phyllites, and as distinguished 

 from cases where decayeii existing felspars are regenerated and 

 re-crystallized, as in the metamorphism of much-weathered igneous 

 rocks and their tuffs and ashes, or from cases in which new felspar 

 is simply an infiltration-product. 



