Microscopical Study of the Belgian Rocks. Bij A. Renarcl. 217 



We obtain for our result a pressure of 66291 mm., or 87 

 atmospheres. 



Another very interesting rock on account of its microscopical 

 constitution is that which was designated by Dumont as hyper- 

 sthenite of Hozemout ; having found that it contains diallage instead 

 of hypersthene, this rock should be called Gabbro. Chemical analysis 

 has demonstrated that here the feldspar is labrador feldspar. In 

 certain cases the thin sections of this gabbro show us the crystals 

 of labrador broken ; the broken parts are slightly separated from 

 one another ; and, what is important for our interpretation, the 

 surrounding minerals and the base present the aspect of a mass 

 bent, as is seen in the rocks of true volcanic character which have 

 a fluidal structure (Fig. 4). The diallage such as we have found 

 here should not be confounded with hypersthene, on account of its 

 want of dichroism and because we find frequently the cleavage cor- 

 responding to /i^ ( oo P GO ) associated with another cleavage per- 

 pendicular to the former. This second cleavage is indicated merely 

 by irregular and interrupted striae which correspond to the plan g^ 

 ( CO P 00 ) (Fig. 5). This second cleavage, as is well known, is less 

 easy than the other. Hence we never have the regular reticulated 

 structure which should be found in augite. This diallage is fre- 

 quently surrounded by Uttle fibres of hornblende 0,3 mm. in length 

 (Fig. 6). This fibrous hornblende is colourless, perfectly trans- 

 parent, and dichroic. The minerals which constitute this rock 

 are imbedded in a greenish substance, which under the Nicol prisms 

 appears in some places monorefringent, and in others presents a 

 sky-blue colour. Upon close examination this substance is found 

 to be of a fibrous structure and ofiers an irregular network similar 

 to that which is well known in the case of serpentine ; although we 

 have not met with olivine whose decomposition would have given 

 the explanation of the presence of serpentine. Besides apatite we 

 have also detected ilmenite, remarkable on account of its decomposi- 

 tion products, and which we will now briefly describe. 



The sections of this titanic iron are surrounded and covered in 

 some cases with coatings of an opaline substance perfectly homo- 



for < = 307 \ +mx = 2,597095795 



rf- = 129,91 

 \ ■{■ mx 



log. F = log. a + -— ^ log. a 



1 + mx 



log. F = r,9590414 + 125,91 x 0,38618275 



= 1,9590414 + 4,86241700 



= 4,82145840 



F = 66291m = 87 atmospheres. 



