366 H. H. Read — The Two Magmas of Strathbogie. 



a mixture of tremolite and serpentine, and in the olivine-rich 

 types homogeneous serpentine usually results. Tlie tremolite of 

 the pyroxene rocks lias been developed under pressure from the 

 solid augite, the large crystals of which are crushed and distorted, 

 the amount of tremolite present being in intimate relation to the 

 degree of crushing suffered by the rock. 



At Portsoy a second independent intrusion of a monomineralic 

 rock is furnished by a small mass of anorthosite. This coarse- 

 grained greenish-grey rock is composed almost entirely of labradorite 

 felspar in crushed augen between excessively fine-grained cataclastic 

 granules of similar felspar. Scarce chlorite, rare epidote, and talc 

 complete the rock. This anorthosite is undoubtedly an independent 

 intrusive mass, and is against Dalradian limestone and schist on one 

 side and a pyroxenite on the other. 



The great majority of Older basic rocks is derived from a gabbro 

 which is found well preserved at Portsoy. This parent rock of the 

 epidiorites and amphibolites is a coarse gabbro composed of labradorite, 

 augite, apatite, and iron oxides, with secondary hornblende and 

 biotite. The rather elongated crystals of labradorite are saussuritized 

 and slightly crushed. The ophitic augite is marginally altered to 

 the fibrous or compact hornblende of the epidiorites. 



Primary modifications of this gabbro are common, but form only 

 local variations in the gabbro mass. By the dwindling of the felspar 

 content and by the incoming of olivine, a rock is formed in which 

 the original constituents were olivine, diallage, and scarce labradorite. 

 The olivine, now entirely replaced by pilitic tremolite, quartz, 

 chalcedony, and carbonates, formed rounded grains poikilitic in large 

 diallage plates which are in process of alteration to hornblende and 

 biotite. The felspar is a scarce rather fresh basic plagioclase. 

 Compound reaction rims between the olivine and felspar are 

 occasionally developed and are exactly similar to those found in 

 certain types of the Younger Series. The rock is occasionally 

 crushed. 



In the Burn of Durn at Portsoy an enstatite gabbro is found 

 in which a colourless enstatite is predominant over the monoclinic 

 pyroxene. Both species of pyroxene are partially or completely 

 altered to a pale-green hornblende, and cataclastic effects are 

 common in the plagioclases. 



Bands and streaks of a rock more acid than the predominant type 

 are found in the gabbro of Portsoy. Such dioritic types are 

 composed of a medium plagioclase, quartz, and biotite. 



Primary banding is common in the Portsoy gabbro. This is of 

 two types, the first being formed by alternations of two rocks, the 

 second by alternations of two minerals. The first type consists of 

 bands of a labradorite rock with a little hornblende secondary from 

 pyroxene, alternating with bands of hornblendic (originally 

 pyroxeuic) rock with subordinate plagioclase. This complementary 

 banding is undoubtedly original. Those rocks to which the name 

 gabbro schist may be applied show a delicate banding of light 

 coloured saussuritized labradorite and of dark secondary hornblende 

 enclosing augen of diallage. This banding is due to movement 



