138 J. J. H. TEALL Oj!T THE METAMORPHOSIS OF 



former section over the polarizer, the vast majority of the grains 

 are seen to change from a rich deep green to a very pale greenish 

 yellow. The green tint appears when the direction of schistosity 

 lies approximately parallel to the plane of vibration, and is due to 

 the fact that the least axes of elasticity are only slightly inclined to 

 the long diameters of the grains. The pale greenish-yellow appears 

 in the opposite position, and is due to vibration parallel to the least 

 axes of elasticity in the different grains. On rotating the section 

 parallel to the schistosity in a similar manner, the grains change, as 

 a rule, from deep rich green to dark yellowish green, the former 

 tint being due to vibrations parallel to the least, and the latter to 

 vibrations parallel to the mean axis. It must be distinctly under- 

 stood that the above orientation of the grains in the rock, though 

 strongly marked, is not by any means rigidly followed. 



In some non-foliated varieties of the rock of these dykes, plates 

 of hornblende of considerable size with perfectly characteristic 

 cleavages occur ; and by experimenting on these it is possible to 

 make out the cbaracteristic pleochroism of the mineral. If we take 

 a, /3, and y* to represent the greatest, mean, and least axes of 

 elasticity respectively, then it may be defined as follows : — 



a = Very pale greenish-yellow. 

 /3=Dark yellowish- green. 

 y=Eich deep green. 



The absorption for rays vibrating parallel to /3 and y is con- 

 siderable, that for rays parallel to a is very slight. 



Next in importance to the hornblende are the pellucid grains of 

 quartz and felspar. These] do not show a trace of external form. 

 The majority of them are simple, but a few show signs of twin- 

 striation. At first I thought these grains were almost entirely 

 composed of quartz ; but the chemical analj^sis shows such a close 

 agreement between the schist and the dolerite that I can hardly regard 

 this view as probable. Some facts will be mentioned later on which 

 throw light on the molecular changes observed in the felspar- 

 substance ; but at present I must confess that I feel considerable 

 doubt as to the possibility of separating the quartz from the felspar 

 in this rock by microscopic examination. 



The titaniferons iron-ore is sometimes seen in the form of long 

 strips which lie in the plane of schistosity, and at other times it is 

 present as small grains which occur as inclusions in the other 

 crystalline constituents. It is sometimes partially surrounded by 

 grains of sphene. Broken prisms of apatite may frequently be 

 recognized. The only other constituents of any note are a few 

 turbid grains of what is in all probability felspar. The rock is 

 therefore a typical hornblende-schist. 



The two extreme modifications of the rock forming these dykes 

 have now been described at sufficient length. Prom a consideration 

 of all the facts of the case, I have been led to the conclusion that 



* The form of the ellipsoid is assumed to be that usually observed in horn- 

 blende. 



