which is represented by the following analysis : 



Si0 2 ... ... ... 58-16 



A1 3 S ... ... ... ... ... 26-66 



CaO ... ... ... ... ... 5-79 



MgO ... ... ... ... ... .05 



Na 2 ... ... ... ... ... G-99 



K 2 1-76 



100-01 



They belong, therefore, to andesine. As the vein is certainly of secondary 

 origin, and connected with the metamorphosis of the dyke, this makes it 

 highly probable that felspars other than albite may arise as secondary 

 products in metamorphosed basic eruptive rocks. 



Monoclinic Pyroxene. The monoclinic pyroxenes are very largely 

 represented in the basic igneous rocks, and, so far as we know at present, 

 they are in every case either the product of crystallization from the molten 

 magma, or the result of the action of the magma on pre-existing crystals 

 (e.g., hornblende). In no case are they known to have been formed in basic 

 igneous rocks by secondary processes acting after consolidation. The 

 monoclinic pyroxenes vary considerably in chemical composition, and, 

 notwithstanding the very large amount of information which has been 

 accumulated, 11 ' we are still very far from a complete knowledge of 

 the molecular composition of the group. According to the view of TSCHERMAK 

 the pyroxenes (except Wollastonite), are isomorphous mixtures of the following 

 molecular groups: Ca Mg Si,' > 6 (diopside-molecule), Ca Fe SLO,. (Hedenber- 

 gite-molecule), Mg A1 2 Si O, Mg Fe 2 Si O e , Fe AL Si O c and Na Fe Si 2 O e 

 (akmite-molecule). DOLTER and others have shown, however, that other 

 groups must be added to the above in order to explain the composition of 

 all the known pyroxenes. 



It is customary to divide the monoclinic pyroxenes into two groups, 

 depending on the presence or absence of considerable quantities of elements 

 in the sesquioxide condition (alumina and ferric oxide). To the first division 

 belong the varieties designated by the terms diopside, salite, malacolite and 

 diallage ; to the second, the common augites. There is, however, no hard 

 and fast line between the different groups, and many diallages contain five 

 and six per cent, of alumina. An important question arises as to the 

 classificatory value of diallage. The distinctive character of this variety is 

 a lamination, often due to the presence of inclusions, parallel or approximately 

 parallel to the orthopinacoid (100). As there is good reason to believe that 

 this lamination is of secondary origin,' 2 ' and as it is certain that it may be 



(1) See DOLTEH. Uber die Constitution der Pyroxengruppe. T.M.M., Neue Folge, 

 vol. II.. 1879. p. 193. 



(2) BISCHOF held the view that diallage is merely altered augite (see Chemical and 

 Physical Geology, English Edition, vol. II., p 334). It was confirmed by STBENO (Gabbro des 

 Harzes. N.J., 1862, 933), who pointed out that one and the same individual could be augite in 

 one part and diallage in another. 



