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PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



[Nov. 6, 



the feldspath ; its crystals are microscopic, and far from clear ; it has 

 a yellow straw-colour very slightly greenish. Sometimes it is de- 

 veloped in a crystal of oligoclase, the form of which it preserves, 

 taking a yellowish colour and a granular crystalline structure. 



I have found that porphyry loses entirely its green colour when 

 treated with boiling hydrochloric acid, before or after calcination ; it 

 is therefore impossible to attribute this green colour to amphibole. 

 This results also from what has been said above. 



I have ascertained the loss by fire of some samples to be — 



1. Porphyry, blackish green with crystals of whitish oligoclase and a little"! j.g^ 



quartz — of Belgium J 



2. Porphyry, a greenish feldspathic paste, with crystals of oligoclase, chlo-1 ^.g- 



rite, quartz, and small deposits of epidote — of Quenast J 



3. Porphyry, a reddish feldspathic paste, containing crystals of oligoclase,! 



deposits of chlorite forming green spots, grains of quartz, and small > 2*10 

 deposits of epidote — of Quenast J 



4. Porphyry, a bluish green feldspathic paste, with crystals of greenish white \ g.Vi 



oligoclase — of Lessines J 



It appears that the loss by fire in the case of the porphyry is gene- 

 rally somewhat greater than that of the feldspath which forms the 

 basis ; this, as might be expected, is on account of the mixture 

 of the chlorite ; sometimes, however, as with the porphyry of Lessines 

 (see above), the loss by fire surpasses that of the feldspath (see p. 6) 

 by some hundredth parts, which must be attributed to the presence 

 of a carbonate. 



I have experimented, with the view of fixing approximatively the 

 mean composition of the mass of the rock, on a sample coming from 

 the first quarry of Lessines. It had a deep green paste in which 

 chlorite was disseminated ; its crystals of greenish white oligoclase 

 were easily detached. A gramme of the sample, calcined and pul- 

 verized, was submitted to hydrochloric acid for twelve hours in order 

 to ascertain the proportion dissolved in the acid ; and I obtained a 

 greyish or nearly colourless residue, its weight being 75 per cent, of 

 that of the sample operated upon. A fourth part of the rock had 

 been dissolved, and I found that the oligoclase had been partially at- 

 tacked, for the solution contained some centigrammes of alkalies. As 

 to the undissolved residue, it was formed of 18*50 of silica, separable 

 by solution in potass, and of 56*50 of entirely unaltered substances. 



The oligoclase of the porphyries being affected by hydrochloric 

 acid, it is seen that the mixed carbonates cannot be exactly deter- 

 mined by the proportion of the bases which are dissolved in this acid, 

 even when the rock has for basis a feldspath rich in silex like the 

 oligoclase. 



The sample of the porphyry of Lessines contained, moreover : — 



Silica 57*60 



Alumina and peroxide of iron .... 25*00 



Lime 3*23 



Magnesia and alkalies 9 '92 



Water and carbonic acid 4*25 



100*00 



