318 J. Parsoiis — Broion Mica developed from Augite. 



surrounding felspar in a manner approaching micropegmatite. 

 (ii) A pale -green augite, irregularly cracked, probably sahlite. 

 It occurs in rounded fragments, which in places have been broken 

 up and separated. Biotite is chiefly developed round the first- 

 named augite, and it occurs only when that mineral is in contact with 

 the felspar, and has been manifestly corroded by it. With ordinary 

 light it is often impossible to say where the passage of augite into 

 mica commences, for its first manifestation is a very faint brown 

 tinting. With crossed nicols the first indication of its development 

 is a marked increase of intensity in the polarization tints, along 

 a narrow zone at the junction ; this presently shows the characteristic 

 mica cleavage, and varies in colour from a very light brown to 

 a dark bright brown. Beyond the latter magnetite is developed. 

 On the other side of the mass of magnetite the augite shows irregular 

 cracks in which the magnetite is deposited, and beyond this the 

 regular cleavage sets by marked iron oxide, as mentioned above. 

 It should be understood that though this order of sequence is 

 apparently constant, yet any stage owing to the irregularities of 

 the decomposition may be locally very poorly shown or even absent. 

 In some cases the augite is completely decomposed, and there 

 remains only a core of magnetite with a corona of mica, darker in 

 colour near it and lighter in contact with the felspar. The pale- 

 green augite does not so readily develop mica as the other variety, 

 though its smaller fragments show in places an abundant mai'ginal 

 growth. 



Much serpentinization occurs along its cracks, accompanied by 

 the formation of a little mica. Both these facts confirm the idea 

 that this is a non-aluminous augite. It is, however, rich in iron 

 oxide, which is largely excreted in the formation of mica and the 

 serpentinization. Apatite is an abundant constituent of the rock, 

 the larger crystals acting as the nuclei round which the augite 

 consolidated. 



(3) A similar formation of dark mica from augite may be observed 

 in Lauvigite, which closely resembles the Svenore syenite described 

 above. The rock is too well known to require detailed description, 

 but in the present connection the following points may be noted. 

 The augite is of two types : (i) pale-brown, slightly pleochroic, with 

 incipient diallagic cleavage; (ii) pale -green sahlite, irregularly 

 cracked, some serpentine and a little biotite forming along the 

 cracks. This is earlier than the first-named augite and in many 

 places is enclosed by it. The felspathic mixture (anorthite, 

 microperthite) has much corroded the augite, with which at its 

 margins it is in places intimately intergrown. Eound the augite 

 mica is developed, the one passing almost insensibly into the other 

 as already described, and the same darkening of the mica and final 

 formation of magnetite occurs as the distance from the felspar 

 increases. The felspathic magma has in places invaded the augite, 

 producing in its centre patches of dark mica which appear at first 

 sight to be isolated inclusions. Here also the mica is only formed 

 with difficulty in the sahlite. 



