218 Transactions of the Royal Microscopical Society. 



geneous, which seems a result of the decomposition of ilmenite. 

 The first stage of this decomposition is represented by the appear- 

 ance of whitish veins running through the mineral ; a second stage 

 exhibits it enclosed in the opaline substance ; finally, the metamor- 

 phosis can be pushed so far that nothing more is visible except a 

 few black specks (Figs. 7 and 8). Its chemical composition has 

 not been determined, laut we have ascertained that it is unalterable 

 by the action of hydrochloric acid, and therefore it is not carbonate 

 of iron, as has been taught by some. We are, however, persuaded 

 that the opinion of Giimbel, who admits that it is not a decom- 

 position product, cannot be sustained. 



In the Cambrian and Silurian beds of Belgium and of the 

 Ardennes we meet with feldspathic rocks having at the same time a 

 schistoid and a porphyritic texture, and which appear to be regularly 

 imbedded in quartzites, slates, and schists. Dumont interpreted 

 this feldspathic rocks as so many dykes injected between the adja- 

 cent layers ; other geologists have admitted that these schisto- 

 porphyric rocks were the result of a metamorphic action exerted 

 at certain points. The stratigraphic study of these rocks in the 

 Silurian of Brabant and their examination both with the naked eye 

 and under the microscope have led us to admit for them an elastic 

 origin. The microscopic characters on which we rely to demon- 

 strate this fact are that the numerous feldspars in the thin sections 

 are all without distinction broken or their angles blunted, and 

 present at both extremities the appearance of fracture. In the 

 same way the grains of quartz are not terminated by crystalline 

 lines which have their regular form (Fig. 9). However, in other 

 places in the same schisto-porphyric rocks we found indications that 

 a part of the quartz has crystallized in situ. This latter mineral 

 with sericite and triclinic feldspar constitute the essential elements 

 of this rock. Hence we arrive at the same conclusion as Sorby, 

 who considers some sericitschiefer of the neighbourhood of Wiesbaden 

 as elastic, and we know besides that the rocks of which we are now 

 speaking have the same identical schisto-porphyric structure and 

 the same composition as those described by Sorby. 



