DISTINGUISHING FEATURES OF THE SERIES. 12 



17. Microscopic examination shows that the similarities in 



macroscopic characters are only the outward 

 Microscopic characters. 



and visible signs of a constancy in structure 

 and mineral composition. The one constant feature in structure 

 is the even-gr 'ained \granutitic (panidiomorphic) character of the con- 

 stituents. The constant feature in composition is the presence of 

 rhombic pyroxene^ which is generally highly pleochroic, approaching 

 hypersthene, or in some cases amblystegite, in composition. 



The above two characters are constant in what I regard as 

 the unaltered forms of the rocks. But in those which show a clearly 

 defined gneissose structure by linear arrangement of the minerals 

 signs of dynamo-metamorphism are sometimes displayed, and pink 

 garnets almost invariably appear. I have said signs of dynamo- 

 metamorphism are sometimes displayed, because, although nearly 

 all varieties show a linear arrangement of the constituent minerals, 

 the frequent absence of all signs of crushing shows that in some 

 instances the crystals were arranged with their long axes at 

 right angles to the direction of maximum pressure before consolida- 

 tion. Whether this unequally distributed pressure was due to actual 

 lateral compression, or to fluidal movements analogous to that which 

 has frequently been recorded in dykes, it is impossible to determine, 

 and, as far as concerns our conclusions as to the origin of these rocks, 

 is of little consequence. 



Characteristic constituents. 

 Besides hypersthene, which is an invariable constituent, and 

 garnet^ which is extremely common, the following minerals are 

 frequently found in members of this series: — 



Quartz.— ■ Blue, grey, or greenish, often with innumerable 

 acicular inclusions. Also intergrown with felspar 

 to produce the so-called "quartz de corrosion " of 

 the French petrographers. 

 Potash felspar. — Often as microcline and frequently micro- 

 perthitic. 



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