﻿1850.] 



DELESSE ON THE PORPHYRY OF BELGIUM. 



7 



be examined with a microscope, it is seen to be formed of small 

 agglomerated spangles of a blackish green colour, which line the 

 spaces left between the crystals of feldspath, as well as the irregular- 

 shaped cavities in the rock. These spangles are microscopic, and at 

 first sight it is difficult to determine to what mineral they truly be- 

 long; but, having extracted some decigrammes of the dark green 

 portions from a fragment of the Quenast porphyry, I found that 

 their loss by fire was 5*29. As we find, by microscopic examination, 

 that these spangles are only mixed with feldspath, which, however, 

 is found in them in a tolerably great proportion, it results therefrom 

 that their loss by fire is still considerably superior to the proportion 

 obtained by previous experiments made on the impure matter, and 

 consequently these spangles are neither mica nor talc, as is admitted 

 by many geologists. With M. Dumont, I consider these spangles, 

 which are very soft, to be a variety of chlorite, which from its green 

 colour, sometimes passing into black, is rich in oxide of iron, and in com- 

 position ought to closely resemble ferruginous chlorite * and ripidolith. 

 Its matrix presents, moreover, the greatest analogy with that of tbe 

 two varieties of chlorite developed in the cells of melaphyres and 

 volcanic rocks, or in the cavities of protogines and talcy rocks. 



Quartz is very frequently met with in the paste of this porphyry. 

 M. Drapierf has observed it in dihexaedral crystals as well as in red 

 quartzose porphyry. This porphyry, however, does not always contain 

 quartz, and according to M. Dumont, this is particularly the case 

 with the variety he discovered at Hosemont ; consequently, when 

 the crystallization of the rock took place, there remained but a small 

 amount of silex, and even this was not the case in all its parts. 



In some samples, either in those of a light colour, or in those of a 

 deep and uniform colour, there are sometimes observed flakes of am- 

 phibole and of green hornblende which are some millemetres long. 



Like most porphyries, the porphyry under consideration contains, 

 mixed up in its paste, carbonate of lime and carbonate of iron ; iron 

 pyrites is also present $ ; in the Lessines porphyry there is also copper 

 pyrites, sometimes crystallized, sometimes amorphous, in nodules of 

 the size of a small nut ; and green carbonate of copper, which is 

 either in small veins in the porphyry, or disseminated in an earthy 

 state in decomposed varieties, which pass into a state of clay. 



Finally, there is found in it, as in all porphyries having a feldspath 

 of the sixth system for a base, small streaks or veins formed of hyaline 

 quartz, sometimes smoke-coloured, of green epidote, and of white 

 spathose carbonate of lime ; and, moreover, in the porphyry of Les- 

 sines violet axinite occurs, presenting the forms " equivalente " and 

 " sous- double " of Haiiy. 



The epidote is here much more abundant than is general in por- 

 phyries ; in the variety from Quenast, for instance, it forms a very great 

 number of small deposits disseminated partly in the paste, partly in 



* Annales des Mines, 4me serie, t. xii. p. 223. 



f Memoire couronne par l'Academie de Bruxelles, t. 111. Coup d'ceil minera- 

 logique sur le Hainault, par M. Drapier, p. 18 et suivantes. 

 X Coup d'oeil sur la Geologie de la Belgique, par d'Omalius d'Halloy, p. 25, 



