﻿1850.] 



DELESSE ON THE PORPHYRY OF BELGIUM. 



9 



This porphyry is but poor with respect to silica, and specially in- 

 ferior to the oligoclase which had been previously analysed : this is 

 accounted for by the presence of the chlorite and carbonates ; besides 

 which, the sample contained no quartz. 



It is clear that the proportion of oxide of iron, of magnesia, and 

 of lime, as well as the loss by calcination, must be more considerable 

 than in feldspath, whilst that of alkalies is, on the contrary, less. 



Although the porphyry of Belgium contains quartz, its proportion 

 of silica is very notably smaller than that of quartzose porphyry pro- 

 perly so-called, which is rarely inferior to 70 per cent. ; it has a 

 basis of oligoclase and contains no orthose, which, on the contrary, is 

 the predominating feldspath of the latter ; consequently these two 

 rocks differ in a very important mineralogical character. 



The porphyry of Belgium, which has just been described, is met 

 with over a small extent on some isolated points ; particularly in some 

 slaty districts. At Quenast it forms a small hill surrounded by slaty 

 schists, in contact with which the porphyry becomes laminated. At 

 Lessines, on the contrary, it divides into prisms. 



The geological map of M. Dumont, accompanying the work of 

 M. d'Omalius d'Halloy on the Geology of Belgium, indicates the 

 localities where this porphyry has been recognised ; besides Lessines 

 and Quenast, there might be cited some other points between Enghein 

 and Nivelles, Pitrel on the Metragne, Hozemont west of Liege, Hen- 

 nurjeres south of Audimont, &c. 



This porphyry is susceptible of being decomposed by the kaolini- 

 zation of its feldspath, and this is particularly observed in its upper 

 parts exposed to atmospheric action ; ultimately it is transformed into 

 a coloured kaoline, and a yellowish brown hydro-oxide of iron, in 

 which some grains only of quartz remain. 



This porphyry, which is employed for paving, is worked in exten- 

 sive quarries ; from which are obtained the paving-stones used in 

 Brabant, in Flanders, and in the greater part of Belgium, as well as 

 in Holland. During the last few years it has had to compete with 

 the brown freestone of Fontainebleau. It furnishes pavements of in- 

 definite hardness, never becoming friable like the brown freestone ; 

 they have, however, the great inconvenience of becoming, when worn 

 smooth, too slippery for horses. 



3. On the Rose-coloured Syenite of Egypt. 

 By Prof. Delesse, Engineer of Mines. 



[Communicated by the President.] 



The rose-coloured syenite of Egypt is formed of quartz, orthose, 

 oligoclase, mica, and frequently also of hornblende*. 



The quartz is hyaline and grey ; it has sometimes a slight violet 



* See also Lieut. Newbold on the Geology of Egypt, Quart. Journ. Geol. Soc. 

 1848, vol. iv. p. 310. 



