420 



reveals itself under the microscope in what is known as undulose 

 extinction. When the limit of elasticity is exceeded the crystal is 

 cracked or broken, and the separate parts may be more or less 

 removed from each other. Very commonly the original minerals are 

 more or less replaced by a micro-crystalline or crypto-crystalline 

 aggregate of finer grain than the original rock. The partial replace- 

 ment of the original minerals by such an aggregate is accompanied 

 by the development of well-marked structural features. The relics of 

 the original minerals are seen to lie in a secondary aggregate. Thus, 

 in the case of a gneissose rock of granitic composition Ave frequently 

 find that the centres of the light-coloured lenticles (see Fig. 1, 

 Plate XLIII.) are occupied by a large irregular grain of felspar. This 

 felspar passes at its margin into a granulitic aggregate. If the section 

 be cut parallel to the greatest axis of the ellipsoid of deformation 

 (direction of stretching) the ends of the lenticle will be seen to tail 

 off into the more evenly-foliated rock, and these tail-like endings will 

 consist of granulitic material. In general, we may say that, where 

 large grains of the original constituents remain, the finer secondary 

 aggregates sweep round them in the manner characteristic of what is 

 called fluxion structure. It very commonly happens that definite 

 planes of movement may be followed through a microscopic slide 

 across the different constituents. When this is the case, these 

 planes of strain are occupied either by the secondary granulitic or 

 crypto-crystalline aggregate, or else by secondary minerals, such as 

 mica (see Plate XLIL). 



When all traces of the original form of the minerals have dis- 

 appeared the entire rock shows a granulitic or crypto-crystalline 

 structure. When mica has been extensively developed it takes the 

 form of a mica-schist. 



In the metamorphosed rocks which show a marked foliation, the 

 orientation of the constituents stands in direct relation to the ellipsoid 

 of deformation, their largest diameters lying parallel with the greatest 

 axis of the ellipsoid. This renders it highly important that sections of 

 foliated rocks should be cut in definite directions. The true character 

 of a schist is best elucidated by preparing three sections ; one parallel 

 to the plane of schistosity; another perpendicular to this plane and 

 parallel to the direction of stretching; and a third perpendicular to 

 the schistosity and also to the direction of stretching. 



A very common structure in deformed rocks in which a marked 

 foliation has been developed is the occurrence of parallel veins and 

 planes of inclusions which cut the foliation at a high angle. Veins 

 of the kind here referred to are seen in Plate XXVI. In rocks con- 

 taining quartz, parallel planes of liquid inclusions may often be seen 

 traversing the slide and cutting the planes of foliation nearly at 

 right angles. These are apparently connected with the extension 

 which the rock has undergone in the direction of the greatest axis 

 of the ellipsoid of deformation. 



