Microscopical Study of the Belgian Rocks. Bij A. Renard. 213 



angles are not perfectly sharp ; and, what is very remarkable, the 

 lines which lie perpendicular to the polysynthetic lamellae run 

 clear through, without any break at the point where they intersect 

 the striae (Fig. 1). 



In this diorit the microscope has enabled us to prove the pre- 

 sence of orthoklas, a result to which we have been led by the phe- 

 nomena of polarized light, just as is observed in the twin crystals of 

 Carlsbad. We need not stop to describe the microscopic details of 

 the hornblende, which is generally much altered and often sur- 

 rounded by an opaque zone in a state of decomposition, which has 

 also obliterated the cleavages. This mineral often contains apatite, 

 and this we consider to have been formed simultaneously with the 

 hornblende ; it is also intimately associated with chlorite, ilmenite, 

 magnetic iron, biotite, epidote (Fig. 2), calcspar, and quartz; 

 minerals which we believe to have been formed for the most part 

 by the decomposition of the hornblende. Notwithstanding its 

 state of decomposition, the sections of this mineral are still 

 dichroic. We have also found in this rock uralite, augite, and 

 diallage ; and by means of the microscope we have proved the 

 presence of crystals of apatite and ilmenite. These two minerals, 

 which play an important part in the Belgian plutonic rocks, are 

 always of microscopical dimensions, and had not been remarked 

 in that country before we examined them under the microscope. 



The microscopic study of the quartz of this rock is of the 

 greatest interest, since it allows us to determine to a certain point 

 the conditions in which this diorit was formed. Mineralogists 

 have long been engaged in the study of the numerous minerals con- 

 tained in quartz, and of the liquids enclosed in the cavities of this 

 mineral ; but it is especially Sorby, who by opening the way to a 

 new method in j)etrography, has shown the geological importance 

 of these phenomena. Following his example, and relying on the 

 facts revealed by the microscope in the cavities of the quartz of 

 this rock, we will endeavour to determine the temperature and the 

 pressure at the moment of the crystallization of this rock. 



The sections of the quartz of Quenast are rich in liquid 

 cavities, but many of them, besides the bubble and the liquid, 

 contain little cubic crystals (Fig. 3). An ellipsoidal cavity has 

 enabled us to measure with great precision by means of the micro- 

 meter the dimensions of the cavity, those of the bubble and of the 

 cubic crystal. 



Major axis of the cavity 0,mm0096i 



Minor axis of the cavity 0,00660 



Side of the cube 0,00214 



Diameter of the bubble 0,00187 



Of all the rocks subjected to microscopic examination, that of 

 Quenast is perhaps, after the syenite of Laurvig, the one which 



