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PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Nov. 6, 



in form ; two or more rows being often placed side by side ; and 

 single rows of cells frequently branch off from the main body. 



Numerous spiculse of sponges are also found, the most common of 

 which are simple, smooth, and pointed at each end. Others are 

 bulbed at one end, and pointed at the other ; sometimes of con- 

 siderable length, but usually broken. I have also found one much 

 branched, quadrifid, with two or more of the extremities bifid. 



In some sections I have observed fragments of a brown colour, ap- 

 parently vegetable, and having a similar structure to portions of Alga. 



2. On the Porphyry of Belgium. By Professor Delesse, 

 Engineer of Mines, &c. 



[Communicated by the President.] 



The constituent feldspath of this porphyry occurs in mackled and 

 finely striated crystals, and necessarily belongs to the sixth system. - 

 The colour of these crystals is white, or slightly greenish white, with 

 a glassy lustre ; and when of a yellow greenish colour, with a fatty 

 lustre. As the hardness of the latter is much less than that of the 

 white crystals, it is probable that they have been altered by infiltra- 

 tion and by pseudomorphosis. When of a red colour, the rubefaction 

 has been produced by atmospheric action. 



I have analysed the crystals of a slightly greenish white tint, and 

 tolerably pure specimens, from the quarries of Quenast. They were 

 easily separated from the somewhat darkish green matrix, the latter 

 containing some grains of quartz. They contain — 



Silica 6370 



Alumina 22*64 



Oxide of iron 0*53 



Oxide of manganese traces. 



Magnesia 1*20 



Lime 1 "44 



Soda 6-19 



Potass 2-81 



Loss by fire 1*22 



99-69 



The constituent feldspath, therefore, of this porphyry is oligoclase. 

 This oligoclase, as in all porphyries, is scattered through a feld- 

 spathic uncrystalline paste, the residue of crystallization, and in which 

 also are found all the substances that are contained in the feldspath, 

 but in somewhat different proportions. I will designate it by the 

 name of feldspathic paste. The green colour of this paste shows 

 that it is richer in oxide of iron and magnesia than the feldspath, and 

 this is probably to be attributed to a pseudomorphose, wbich would 

 tend to transform certain portions ; indeed, if the dark green portion 



