456 ME. A. HAEKEK OS THE EKTJPTIYE EOCKS IN THE 



ascribed to the increase in bulk of the grains consequent upon the 

 process of serpentinization*. 



Felspar, when it occurs in the hornblende-picrite, is either in 

 small slender crystals, simple or once twinned, or in large irregular 

 plates enclosing olivine. The extinction-angles observed agree 

 with anorthite. The mineral is probably always of earlier con- 

 solidation than the augite, and is moulded by the plates of horn- 

 blende. 



The augite is of the usual very pale brown tint or almost colour- 

 less, and has the prismatic cleavage well marked. It is almost 

 always in the form of irregular plates, or forms a core to the horn- 

 blende. In rare cases, however, it has crystal-outlines, the cross 

 section being a regular octagon due to the equal development of the 

 prism and pinacoids. The hornblende is of the same rich brown 

 colour as in the hornblende-diabases, but this passes occasionally 

 into green, which gives, for vibrations parallel to the axes of 

 elasticity : — a, very pale brown ; /3, pale olive-green ; y, rather 

 pale grass-green. The brown hornblende also passes, in places, into 

 a colourless variety. The usual prismatic cleavage is well seen in 

 the slides, and occasionally a cleavage parallel to the clinopinacoid. 

 Twinning on the orthopinacoid is only rarely seen. 



The hornblende never shows idiomorphic boundaries, and it 

 usually occurs in close relation with augite. Either a plate of 

 augite has a partial or complete border of hornblende, or a horn- 

 blende plate encloses a core of augite, the boundary between the 

 two being often exceedingly ragged and labyrinthine. The horn- 

 blende and augite of each plate have the vertical or c-axis and the 

 plane of symmetry common, but are apparently in reverse position 

 to one another. This appears from the fact, verified in several 

 instances, that in a clinopinacoidal section the extinction-angles are 

 about 40° for the augite and 20° for the hornblende on the same side 

 of the cleavage-traces. Some slides show little or no augite. The 

 relations of the two minerals are, on the whole, in accord with the 

 idea that the bulk of the hornblende is a later formation at the 

 expense of augite ; but it is not safe to conclude that this is so of all. 

 Indeed, that some of the hornblende is original, is evident from the 

 fact that it not unfrequently encloses distinct grains of augite 

 without any definite crystallographic relation. 



A brown mica with the characters of biotite is almost always 

 present, and sometimes in such quantity as to give a distinct 

 varietal character to the rock in which it occurs. Normally the 

 mineral is brown, with the usual intense dichroism, but it often 

 becomes paler and even almost bleached. It also passes at the 

 edges of the fl.akes into a mineral which gives a grass-green colour 

 for vibrations parallel to the cleavage-traces and a golden-brown for 

 the perpendicular direction : this seems to be the substance which 

 Mr. TeaU identifies as chlorite f. The biotite has two modes of 

 origin. It is in part original, being then usually later than the 



* Cf. Judd, Quart. Journ. Geol. Soc. vol. xhi. p. 86 (1886). 

 t British Petrography, p. 98, 1888. 



