56 Diorites and Granites of Swift's Creek, 



other alternate for a short distance with other contact sedi- 

 ments. They are composed mainly of crystalline grains of 

 felspar and quartz, and have often, as an accessary consti- 

 tuent, a silvery or greenish muscovite mica. This last is 

 locally so abundant that a muscovite granite results. The 

 felspars are mostly triclinic. The quartz fills in interspaces 

 as granules, but other spaces are filled by small crystalline 

 grains of felspar. Such a rock resembles in some respects 

 a granulite of the regional metamorphic series, but it differs 

 in having a more granitic character, in having potash mica, 

 and in wanting the constituents schorl or garnet which are 

 characteristic of the granulites of the neighbouring regional 

 schists. The most appropriate term for such rocks is a/plite — 

 a semi-granite. They may be regarded as being in the con- 

 tact series the analogues of the granulites of the regional 

 series of metamorphic schists. 



Athough in hand specimens these rocks could scarcely be 

 suspected to be altered sediments, yet the examination of 

 them in the field left no doubt in my mind as to their 

 original character, nor that their present condition is due to 

 the complete molecular re-arrangement and metamorphism 

 of the sediments. 



I now come to speak of the next lying zone — hornfels. 

 These rocks fall naturally into two groups, the quartzose and 

 the quartz free, representing the sandstones and slates of 

 their former condition. The paste which more or less 

 cemented the quartz grains of the former probably resem- 

 bled the main mass of the latter. The quartzless samples 

 of hornfels which I examined I find to consist of a ground- 

 mass, and also of what may be called pseudo-nodules. The 

 alterations are all micaceous. The groundmass consists of 

 minute flakes of some colourless micaceous mineral which 

 either forms aggregates of crumpled folia or else which are 

 to a certain extent parallel, and then simulate a homogeneous 

 crystal by becoming dark simultaneously when rotated between 

 crossed nicols. The pseudo-nodules often approach the true 

 nodules in character by having narrow margins of mica plates 

 separating them in part from the remainder of the mass. I 

 have usually found that there was a considerable amount of 

 black amorphous substance in flakes in these spots, and 

 this appears to be a form of carbon surrounding these 

 pseudo-nodules. The ground mass consists of two kind of 

 material; first, micaceous aggregates, or else small over- 

 lapping plates of mica, with larger plates interspersed in the 



