1835*] Porphyritic Greenstone Rocks. 273 



in front, upon the second group behind. This growth, 

 which consists of many individuals, is often repeated ; the 

 applied faces changing frequently before and behind with un- 

 even fracture, so that, in this manner, a single grain of albite 

 often consists of a great number of regular united indivi- 

 duals. But the cleavage faces of albite, in the different 

 varieties of diorite, are not equally perfect ; on the whole, 

 they are not so perfect as those of felspar ; but frequently, 

 the fracture is found extending in other directions, and is 

 fine splintery. 



The albite is white, commonly only translucent, and trans- 

 lucent on the edges ; frequently it is greenish white, probably 

 coloured by portions of hornblende, and this especially 

 occurs when the cleavage faces appear indistinct. 



The hornblende is doubly cleavable ; the faces cutting each 

 other at an angle of 124°. It is greenish black, or blackish 

 green, and opaque. Before the blow-pipe it melts, without 

 frothing, into a black glass, which is slightly magnetic. 



As accidental constituents, the rock contains : — 



Quartz, — in grayish or milk-white grains, with more or 

 less of a greasy lustre. 



Mica, — in greenish, or pinchbeck-brown scales. 



Iron Pyrites, — in small single hexahedrons, and in small 

 fine interspersed portions. 



Magnetic Iron Ore, — in small fine interspersed pieces. 



The proportions in which the principal constituents occur 

 are different. It seldom happens that albite and hornblende 

 are found in equal quantities in diorite; usually one or 

 other constituent predominates, especially the hornblende ; 

 in which case the diorite possesses a very black appearance, 

 and the albite appears greenish-white, and very cleavable. 

 Quartz, mica, and the other accidental constituents, are met 

 with in subordinate proportions. 



The size of the grains of the constituents varies as much 

 as their proportions. The mixture is sometimes large 

 granular, as in the diorite of Konschekowskoy Kamen, near 

 Bogoslowsk, in Ural, where the crystals of Hornblende are 

 frequently above an inch in length. Similar large grained 

 diorite exists in the Royal Mineralogical Museum, which 

 was obtained by Humboldt, from Italy. 



More frequently the diorite consists of moderate sized 



VOL. II. T 



