126 W. M. Sutchhigs — An Interesting Contaci-Roeh. 



going on, but under special modifications which are not readily 

 explicable. The solution of original materials has taken place very 

 completely at the outer parts of the nodules, so much so that here 

 even quartz-grains have disappeared and we have areas wholly made 

 up of the product of the solution. Here also we see a gradual and 

 unmistakable crystallizing out of felspar and quartz in this material, 

 in an even more striking manner than in the main mass of the 

 rock, because we can here more distinctly trace the passage of this 

 isotropic base into an aggregate of different minerals. 



It may be noted that tlie formation in these nodules, and else- 

 where in the rock, of isolated patches of anthrophyllite and andalusite, 

 tends to the conclusion that the inter-action of the components of 

 the shale was not limited to such very short distances as have been 

 inferred by observers in other cases (see Harker and Marr, Quart. 

 Journ. Geol. Soc, vol. xlix. p. 368), but that a more considerable 

 amount of interchange and transfer of materials has taken place. A 

 similar opinion was expressed by Miss Gardiner in respect of 

 contact-rocks at New Galloway (see Quart. Journ. Geol. Soc, vol. 

 xlvi. p. 579). The analysis of a hand-specimen from which some 

 of the sections described above were cut gives — 



Silica 



Alumina , 



P'erric oxide 



Magnesia - 



Potash 



Soda 



"Water ... 



100-44 

 We can now consider other series of slides and see other phases 

 of the same processes. As regards the main mass of the rock, 

 we have variations in the relative amounts of the components, 

 and of their present condition. At some parts there is much more 

 mica and very little of the mosaic of good-sized grains of quartz and 

 felspar forming, with little or none of the isotropic matter. The 

 amount of clastic quartz, and the size of the grains, varies also, and 

 so does the degree to which they are attacked and corroded. Other 

 specimens may be obtained of which slides show that they consist 

 in the main of a fine-gi'ained aggregate of quartz and felspai*, 

 passing down into a quite cryptocrystalline felsitio-looking mixture, 

 and opening up, on the other hand, here and there into numerous 

 clear and glassy patches, which in polarized light are seen to 

 consist wholly of groups of well-twinned plagioclase, which can 

 be identified as albite in many individuals, whilst the extinctions 

 of some others point to the probable presence of a species allied 

 to oligoclase. 



These very fine-grained mixtures of quartz and felspar (mainly 

 albite) are a frequent product of the alteration of shales or slates 

 by dolerites (diabase), and are fully described by authorities writing 



1 A part is present as ferrous oxide — hence excess in the analysis. 



