2i8 INCLUSIONS AND CONCRETIONS IN GRANITE. 



Inclusions in Granite generally have an irregular polyhedral form. 

 They usually belong to the schistose rocks. The larger fragments 

 are sharply angular, but many are rounded, like rolled stones and 

 boulders, some of which, according to Zirkel, have a length and 

 breadth of many thousand feet ; one of the largest occurs between 

 Carlsbad and Eibenstock. Sometimes granite veins pass through 

 these inclusions ; one such, first instanced by Dr. Forbes, is seen in 

 Whitesand Bay at Land's End. 1 Sometimes the smaller included 

 fragments, which often lie parallel to each other, are so abundant as 

 almost to give the granite the aspect of a breccia. In the Fichtelge- 

 birge, near Reizenstein, the granite is so blended with clay slate as to 

 form a fine brecciated mass, and as a rule the included fragments 

 belong to the neighbouring rocks ; and varying with the locality, 

 comprise gneiss, mica schist, clay slate, &c., which have undergone a 

 further metamorphism. 



Limestone has been found in granite in the Pyrenees, and can be 

 readily identified with the deposit from which it was derived. These 

 fragments have a crystalline structure, which is better developed on 

 the surface than in the interior ; and while the latter retains its dark 

 colour, the surface for the depth of an inch is a snow-white marble. 



Zirkel remarks that these inclusions must be carefully distinguished 

 from the small concretions in granite, which usually have a finer 

 texture, and abound in mica. 



Concretions in Granite. Mr. John Arthur Phillips observes that' 

 a concretionary patch generally resembles an enclosed pebble, and 

 usually has the outline clearly defined. Such masses, well seen in 

 granite buildings, often enclose crystals of felspar similar to that in 

 the surrounding granite. 



In the granite of Lamorna, fine-grained black inclusions are very 

 abundant ; they are sometimes traversed by grains of the surrounding 

 granite, and may also contain large crystals of quartz. The felspar 

 is partially orthoclase with a considerable proportion of plagioclase. 

 The orthoclase often encloses grains of quartz and patches of triclinia 

 felspar. The quartz contains the usual fluid cavities ; the mica is 

 chiefly dark brown, and often penetrated by well-formed crystals of 

 magnetite. This granite contains light-brown tourmaline and a few 

 small crystals of apatite. 



The pebble-like masses when examined under the microscope are 

 found to be a granular mixture of quartz with felspar and an abun- 

 dance of dark mica ; but the plates of mica are mostly parallel. 

 Imperfect garnets are sometimes found in these concretionary patches. 



The grey granite of Penryn is almost free from patches of this 

 kind, and they have not been observed in the granites of St. Austell. 

 At Gready, in the parish of Luxulian, the granite contains spheroidal 

 bodies which resemble water- worn pebbles of fine-grained greenstone. 

 These concretions show the black and silvery-white micas, often 

 inter-laminated, but differ chiefly from the granite in a finer grain, and 

 greater abundance of black mica. On analysis the chief difference 

 1 "Treatise on Primary Geology," Henry S. Boase, 1834. 



