58 Diorites and Granites of Swift's Creek, 



noted in the quartzless hornfels. The interspaces are 

 occupied by aggregates of minute plates of some colour- 

 less mica, which reacts, like muscovite, where larger plates 

 occur, and can thus be tested. Besides these there is 

 always a considerable amount of brown dichroic magnesia 

 mica interposed between and around the quartz granules ; 

 and the individual size of the magnesia mica is, as a rule, 

 more considerable than that of the muscovite. The most 

 remarkable alteration products are vast numbers of micro- 

 scopic, round, oval, rectangular, and irregularly shaped plates, 

 which cling to the surface of the quartz grains, and even 

 seem often to be implanted therein. I have observed more 

 than once where a swarm was situated apparently near the 

 centre of the quartz grain, that I could cause a succession to 

 appear by altering the focus of the object glass, without, 

 excepting occasionally, being enabled thereby to detect any 

 capillary flaw connecting them. 



The alterations, therefore, which I observe in the contact 

 schists are, outside of theaplite zone, wholly micaceous. In 

 the conversion of the aluminous portions of the sediments 

 into mica, the silica which would be set free seems partly to 

 have been deposited surrounding the original quartz grains, 

 and partly to have formed aggregates of crystalline grains in 

 the less highly altered varieties. In the true hornfels, which 

 may be regarded as one of the former quartzless sediments, 

 the silica seems to pervade the whole mass, and probably 

 helps to give the rock its peculiarly hard and, with the 

 mica, its tough character. Whether the magnesia mica 

 derives its magnesium from the sediments or from solutions, 

 percolating the strata during their metamorphosis, I am not 

 able, in the absence of comparative analyses, to conjecture, 

 and must leave undetermined. 



The hornfels group, besides consisting of quartzose and 

 quartzless varieties, also includes compact and schistose 

 members — the former close-grained, heavy, dark grey, nearly 

 black or bluish black in colour; the latter having much re- 

 semblance to the structure of a fine-grained gneiss, with this 

 exception, that the alternating folia are mainly quartz and 

 mica. This latter variety is often extraordinarily contorted. 



The changes which I have sketched become gradually less 

 and less marked as the distance from the contact increases, 

 until at from twenty to sixty chains the rock masses have 

 only the character of those indurated sediments which I 

 have already described* 



* See ante, p. 20. 



