FEAGMENTS OF OTHER EOCKS CONTAINED IN GEANITE. 9 



When thin sections of Foggen-Tor granite are examined under 

 the microscope, they are all seen to consist of orthoclase, quartz, 

 dark-brown mica, schorl, and a little silvery-grey mica ; a plagio- 

 clastic felspar is also present ; but no magnetite was observed. It 

 may, however, be remarked that this rock, like some of the granites 

 in the neighbourhood of St. Austell, is occasionally marked by red- 

 dish-brown spots, apparently resulting from the oxidation of some 

 mineral containing ferrous oxide. 



A section of the fine-grained patch enclosing crystals of quartz 

 and a rounded crystal of orthoclase, when examined under the 

 microscope, is seen to be composed of the same minerals as the sur- 

 rounding rock, the only difference being in the dimensions of the 

 component crystals and the presence of a larger proportion of nearly 

 black mica. 



This is, therefore, simply a nest of darker-coloured granite, and 

 is probably a concretionary product, although it has been ascer- 

 tained, by cutting through the specimen, that the large rounded 

 crystal of orthoclase does not extend beyond the darker fine-grained 

 patch into the normal granite. This granite sometimes contains 

 large druses or " vughs " lined with quartz crystals, while it oc- 

 casionally becomes decomposed into a soft kaolin, in which disso- 

 ciated silica is imbedded in the form of aggregated crystals of nearly 

 transparent quartz. 



The well-known granite of Shap in Westmoreland, which is 

 remarkable for its beautiful crystals of pinkish-red orthoclase, and 

 is extensively wrought for ornamental purposes, encloses numerous 

 rounded patches of a darker colour than the surrounding rock, 

 which, at first sight, present the appearance of fine-grained inclusions 

 of trap. This granite, in hand specimens, is seen to consist of a 

 ground-mass of quartz, felspar, and black mica, porphyritically en- 

 closing large crystals of red orthoclase ; but, when examined under 

 the microscope, it is found to contain, in addition to those minerals, 

 magnetite, titanite, a little apatite, hornblende, and occasionally 

 some triclinia felspar. 



The patches in this granite are well defined, and generally more or 

 less rounded in outline, varying in size from that of a pea to that of a 

 water-melon. They are commonly much finer in grain than the 

 surrounding rock ; and, on account of the presence of a larger pro- 

 portion of black mica, are, in a great majority of cases, considerably 

 darker in colour. In some instances a portion of the mica included 

 in these dark patches would appear to have become segregated 

 from the surrounding granite, which, in the immediate neighbour- 

 hood of the inclusion, is comparatively free from the presence of 

 that mineral. 



The rounded pebble-like bodies which occur in this granite fre- 

 quently enclose crystals of the beautiful red orthoclase characteristic 

 of the surrounding rock ; but these are generally imperfect in form 

 and have their angles considerably rounded. 



Fig. 3, PI. I., represents a sharply defined inclusion in the Shap 

 granite, scale four-tenths natural size, which, in addition to uumc- 



