22 Diorites and Granites of Swift's Creek, 



almost the whole series, from silky, wrinkled, argillaceous 

 schist to granite. To deal with all such schists as occur 

 within the area I have mapped would be, in fact, to open up 

 the whole question of regional metamorphism, and this is 

 outside the scope of the present paper. I have, therefore, 

 only examined those nodular, argillaceous, and quartz schists 

 which form the fringe of the regional metamorphic area. 



In speaking of the component minerals of these rocks, I 

 must include the ill-defined substance which I have already 

 spoken of as forming the main mass of the slaty rocks and 

 the cement of the sandstones. 



In a slice of the nodular schist from Swift's Creek, 

 examined under the microscope, the ground mass is entirely 

 composed of plates of mica, which are approximately parallel 

 to each other. As, however, the foliations of mica are more 

 or less contorted, the aggregations of mica plates seen in the 

 thin slice are discordant ; some are flat, and some on their 

 edge. The mica plates envelope the nodules, and are, where 

 nearest to them, rather smaller than elsewhere. In treating 

 a thin slice of this rock with concentrated hydrochloric 

 acid for several days, the only perceptible effect was the 

 removal of much iron by which the rock was stained. 

 Among the colourless mica, which I regard as muscovite, are 

 numerous flakes and groups of flakes of dark-brown 

 magnesia mica. These flakes are approximately rhombic in 

 form, are strongly clichroic, although not, perhaps, in so 

 marked a manner as is much of the magnesia mica of the 

 diorites. 



There are throughout all this ground mass innumerable 

 minute, thorn-like, dark-brown to black microliths, which 

 cross each other and interlace in the utmost confusion. I 

 observed that the greatest numbers were associated with 

 the magnesia mica rather than with the potass mica. These 

 microliths have often ragged or brush -like terminations, and 

 suggest that they may be built up of a number of fibres. 

 I am unable further to determine what they are, owing to 

 their minute dimensions, but it has seemed to me possible 

 that they may belong to amphibole.* 



I have observed several instances of alternation of laminae 

 of potash and of magnesia mica, and also frequently the 

 inclusion of minute plates of the former in the latter. In 

 the muscovite of the ground mass I observed minute cavities, 



* See Zirkel MiJcroskopische Beschaffenheit der Mineralien, 1873, p. 491. 



