W. M. Eufchings — An Interesting Contact-Rock. 125 



mediate stages may be seen in which the needles of felspar and 

 quai'tz are beginning to form in the isotropic matter, and so on down 

 to a stage where the patches just begin to pass from the isotropic 

 state into a development in which, they commence to show a faint 

 and speckly polarization. 



In some of the nodules large amounts of anthrophyllite needles 

 have formed in the patches, and a little andalusite is also seen. 

 These fields of isotropic matter, and the new formations of minerals, 

 are seen in these special slides, mainly at the outer portions of the 

 nodules, and at these parts there are none, or very few, of tlie 

 original clastic quartz-grains, while such as are there are much 

 corroded away, and often represented only by small residues. 



In the inner portions the clastic grains are as abundant and as 

 regularly diffused as in the main mass of the rock, while the new 

 formations are much less, or are absent, except that a little isotropic 

 matter lies in among and around the grains, and here and there 

 a little new mosaic is formed. 



Full study of these sections gives the impression that we are 

 here looking at a very interesting and instructive stage of the 

 metamorphism of a shale. The finer-grained material has been 

 completely afifected by the re-solution of a large part of its original 

 constituents, and out of the matter so formed a mosaic of quartz 

 with felspar is seen in process of crystallizing. The process was 

 brought to a close before such re-crystallization was complete, and 

 so we have left for our observation a large proportion of the residual 

 substance, which we may regard as representing the indefinite 

 product of solution, or of aqueous fusion, of some of the original 

 constituents, which we have such good reason to conclude takes 

 place in the processes of contact-metamorphisra. 



In the larger quartz-grains we see that this process of solution 

 was attacking them to a considerable degree, and had it continued, 

 would have completely dissolved them away. That we can here 

 see all this in progress, as it were, is due to the fact that we are 

 dealing with the effects of only a moderate bulk of igneous rock, 

 and to other favourable circumstances. If, for instance, the meta- 

 morphism were due to a large mass of granite (or to a similarly 

 large mass of dolerite), the eai'lier conditions which caused the 

 solution (or aqueous fusion) of the constituents would have lasted 

 long enough to destroy the clastic quartz-grains to a far greater 

 extent, or entirely ; and the conditions during which the re-crystal- 

 lization went on would have also lasted longer, and we should have 

 had a completed '•contact-mosaic" with either no residual "base" 

 or only a small amount of it, and this not, or only to a small 

 extent, isotropic, such as we may see in the aureoles round some 

 granites. 



In the nodules, as above described, we have the same process 

 demonstrated that this is not necessarily so, as felspar fibres may also be positive, 

 though it is more rare. So that though negative fibres are felspar only, positive 

 fibres may be either felspar or quartz, and the relative degree of bi-refraction must 

 assist in distinguishing as far as possible. (See full resume of this subject iu 

 Zirkel's Petrographie, vol. i. 1893, pp. 474 and 476.) 



