348 J. J. Harris Teall — Hornblende-Bearing Rocks. 



of successive layers. Twinning according to the albite, pericline 

 and probably also the Carlsbad laws occurs ; but it is not very con- 

 stant or uniform. These felspars also evidently possessed inclusions 

 arranged in a more or less zonal manner ; but the original nature of 

 these inclusions is difficult to make out in consequence of alteration. 

 It is a noticeable fact that the angles of the inner zones are mostly 

 rounded, although the external faces of the crystals meet in perfectly 

 sharp angles. In some specimens in which the felspars of the first 

 consolidation have been so altered as to have lost to a very great 

 extent their definite optical characters, irregular scales of a vividly 

 polarizing mineral are seen to be scattered through the crystals. 

 These scales often have their long axes arranged in a zonal manner, 

 evidently corresponding with the original structure of the crystal. 

 When examined with a high power, they are seen to possess one 

 strongly-marked cleavage, and to extinguish parallel with this cleavage, 

 so that they may be referred with confidence to a white mica, pro- 

 bably paragonite. The felspars of the ground-mass form together 

 with quartz a microcrystalline aggregate. 



In a large number of rocks only one generation of felspars can be 

 detected. When this is the case, the individuals occur as irregular 

 grains, sometimes of considerable size. Mutual interference appears 

 to have prevented the development of definite form. Twinning on 

 the albite plan is sometimes present and sometimes absent. In all 

 cases the felspar- aggregate plays the role of ground-mass, and horn- 

 blende crystals lie in it, without any regard to the orientation of the 

 individual grains. The coarseness of this aggregate varies between 

 wide limits, sometimes it is so fine as to merit the term microcrystal- 

 line, at other times it is so coarse as to remind one of the structure 

 of a medium-grained granite. In. some of the thin intrusive sheets 

 the^ felspars tend to assume the form of small columnar crystals, 

 which give narrow lath-shaped sections, but even in these cases the 

 outlines are not well defined. 



Pyroxene. — This mineral is not constantly present. It is pale green 

 when examined on a fractured surface of the rock, but perfectly 

 colourless in thin section. It occurs in somewhat imperfect crystals 

 and also as grains and granular aggregates. The forms of the 

 crystals appear to be those characteristic of the common rock-form- 

 ing augites. The maximum extinction in the prismatic zone is about 

 40°. The only cleavages recognized are those of the prism. The 

 mineral has been especially noticed in the rocks which are intrusive 

 in the dolomitic limestone ; but it varies very much in quantity even 

 in these, sometimes being absent altogether, and at other times occur- 

 ring more abundantly than the hornblende. In some of the coarse- 

 grained rocks in which the hornblende attains a considerable size it 

 occurs as inclusions in this mineral. As a good deal has been said 

 about the secondary origin of the rock-forming hornblende, it may 

 be as well to state that no doubt whatever can exist as to the dis- 

 tinctness of the hornblende and the augite in these rocks. Fractured 

 crystals of beautifully-zoned hornblende occur in the porphyritic 

 varieties, and this must be regarded as absolutely conclusive in favour 



