212 Transactions of the Royal Microscopical Society. 



II. — Some Besults of a Microseojneal Study of the Belgian 



Plutonic Rocks. By A. Eenard, S.J. 



{Read before the Eoyal Microscopical Society, April 5, 1876.) 



Plate CXXXVII. 



This paper is a very brief and comprehensive statement of a few 

 of the results obtained by applying the microscope to the study 

 of some Belgian rocks; they are developed more at length in 

 the Memoire which I made together with the Prof, de la Vallee 

 Poussin.* 



The most important of the Belgian plutonic rocks is the quartzi- 

 ferous diorite, found at Lessines and Quenast in the Silurian layers 

 of Brabant. Delesse, who has made a chemical study of it, con- 

 sidered the base of that rock as a residue from crystallization, i. e. 

 as a silicate whose variable elements are silicic acid, and all the 

 bases which are found in the various minerals porphyrically de- 

 veloped in this rock. This eminent lithologist extended therefore 

 to the rock in question his opinion concerning the constitution of 

 the base of porphyries in general. But the microscope enables us 

 to see that in the rock of Lessines and Quenast the base is com- 

 posed of grains of quartz and feldspar forming a micro-granitoid 

 agglomeration. As for the oligoclase whicb, together with quartz 

 and more or less altered hornblende, makes up the essential elements 

 of this rock, it presents a structure already observed in the plagio- 

 clases of trachytes and andesites. This structure is remarked in 

 lines which are parallel to the outlines of the crystals. With 

 ordinary light, these lines are of a feeble brownish colour ; their 



EXPLANATION OF PLATE CXXXVII. 



Fig. 1. — Oligoclase in the diorite of Quenast, showing a concentric structure. 



Polarized light, x 40. 

 2. — Epidote and calcspar, Quenast, X 40. 

 3. — Liquid cavities containing crystals of Na CI enclosed in quartz, Quenast. 



The prismatic crystals are tourmaline. X 450. 

 4. — Crystal of labrador fractured and bent by fluidal structure in gabbro, 



Hozeraont, x 40. 

 5. — Diallage in gabbro, Hoze'mont, showing the cleavages of that mineral, 



X 25. 

 6. — Diallage surrounded by fibrous hornblende, x 40. 

 7 and 8. — Ilmenite, covered in some parts by its characteristic white coating, 



in gabbro of Hoze'mont, x 25. 

 9. — Fragments of crystals of plagioclase and quartz, surrounded by 8ericite 



in the elastic Porphyroid of Pitet. Polarized light. X 120. 



* " Mem. sur les Caracteres Mine'ralogiques et Stratigraphiques des Roches dites 

 Plutouiennes de la Belgique et de I'Ardenne Fran9aise," par Ch. de la Valle'c 

 Poubsin et A. Renard, S.J., t. xl. des ' Mem. Couronne's de I'Acade'mie do Belgique.' 

 This work, which is now in the Press, will soon appear. 



