272 Gustav Rose on Greenstone and [Oct. 



Article IV. 



On the Rocks which are distinguished by the names of Green- 

 stone and Greenstone Porphyry. By Gustav Rose.* 



The rocks which in geology are distinguished by the names 

 of greenstone and greenstone porphyry, possess very various 

 mineralogical characters. There appear to be five different 

 species, which may be provisionally termed Diorite, Dioritic 

 porphyry, Hypersthene rock, Gahhro, and Augite porphyry. 

 These possess the following characters : — 



1 . Diorite, — A granular mixture of albite and hornblende. 



The albite, as it occurs in this rock, is generally charac- 

 terized by two cleavages, (P and M), which cut each other 

 at an angle of about 93°. The first face of cleavage P 

 exhibits the resulting angle, which is so characteristic of 

 albite when compared with felspar ; the edge goes parallel 

 with the second face of cleavage, and by a twin increase, 

 the granular increments proceed parallel from the second 

 cleavage face.-f- 



Frequently this combination is repeated ; a third individual 

 being applied on the second, and a fourth upon the third, 

 &c., so that the third individual has with the first, the fourth 

 with the second, and also the alternating individuals, among 

 each other, a smooth stratification. Where, therefore, as 

 in such cases commonly happens, the individuals of one 

 layer predominate, they appear as one individual, which is 

 applied more or less strongly to the first cleavage face, 

 parallel with the second. Such groups of individuals occur, 

 combined with each other in accordance with the manner 

 in which two crystals unite in the Carlsbad twin felspar. 

 They are likewise united with a second cleavage face ; but 

 the first, (here always applied faces) lies upon one group 



* From Poggendorff 's Annalen, xxxiv. i. [This paper, which is one of great 

 value, deserves an attentive consideration from geologists. — Edit.] 



t Rose states that albite, although found massive, is always radiated, never in 

 laminae, which distinguishes it essentially from felspar. The planes of cleavage 

 are situated on the same side, but in felspar on opposite sides in the two crystals. 

 Sometimes the crystals are united by their planes M , and, consequently, have their 

 planes on diiferent sides ; but then, the two crystals are attached by their other 

 faces, to other crystals, in the usual way. — Gilbert's Ann. der Physik, 1823, 

 St. 2.— Edit. 



