360 The Lhuydmor JBdch Granophyre. 



compared with the glomero-porphyritic aggregates in a lava. A 

 colourless non-pleochroic augite is abundant, and is frequently repre- 

 sented by secondary products ; the freshest augite occurs near the 

 centre of the ferro-magnesian aggregates. The commonest alteration 

 is to a fibrous uralitic amphibole with pleochroism, bluish parallel 

 to c, strong green parallel to h, pale yellowish-green parallel to a. 

 The fibres of this uralite frequently penetrate the surrounding 

 crystals. In many cases a core of augite is present in 

 crystals of amphibole. Hydrated iron oxide is present in the cracks 

 in the augite, and a slightly pleochroic chlorite with an extremely 

 low birefringence is a common alteration product of the pyroxene. 

 Occasionally a sahlitic striation is observed in the centre of the 

 augite crystals. 



An interesting feature is the presence of elongated idiomorphic 

 prisms, consisting mainly of uralite and bastite ; these are often 

 intergrown with the augite, and apparently are pseudomorphs 

 after rhombic pyroxenes, although no fresh rhombic pyroxenes 

 have been observed. 



Apatite, in elongated prisms, is unusually abundant in the basic 

 patches. Some of the prisms show long central cavities. Magnetite 

 and titaniferous magnetite are abundant in the " clots ", though 

 the granules are seldom found outside the basic patches. Many of 

 these iron ores are enveloped by a pellicle of chlorite. Epidote occurs, 

 generally in association with the ferro-magnesian minerals. Biotite 

 and hornblende of a primary nature have not been observed. 



III. The Granite. 



Near the edge of the laccolith, the micropegmatite disappears 

 almost entirely, and the rock becomes a normal granitic rock. In 

 hand specimens, it appears greyish, with the ferro-magnesian 

 constituents more evenly distributed than m the granophyre 

 described above, although small basic patches do occur. The pinkish 

 colour of the micropegmatite is, of course, absent from the hand 

 specimen of this marginal type. Fresh specimens are rather difficult 

 to obtain, though under the microscope most of the constituents 

 may easily be identified. 



A characteristic feature of this marginal rock is the presence of 

 relatively large amounts of interstitial quartz, undoubtedly primary, 

 containing numerous inclusions of earlier formed minerals, amongst 

 which needles of amphibole are particularly abundant. 



The felspars are mainly of idiomorphic habit, similar to those 

 described as occurring in the granophyric type ; the polysynth etic 

 twinning may be observed. The outer zones are very much 

 weathered, but probably consist of the microperthite described 

 above. G-ranophyric structure is almost entirely absent from this 

 marginal type of the laccolith. 



In contrast to the granophyric type, augite is wanting, and the 

 crude glomero-porphyritic structure of the ferro-magnesian minerals 



