227 



evidently developed during the growth of the crystal, and is a consequence 

 of the fact that less material is added in the direction of the macro- 

 diagonal than in directions at right angles to this. Indications of the 

 prism faces (110 and 110) may be observed in the zones of inclusions, but 

 the angles are more or less rounded as is the case with the angles of the 

 crystal itself. 



In addition to the large felspar-crystals there are small green spots 

 which represent pseudomorphs after a pyroxene. Under the microscope 

 crystals of augite, often twinned, may also be occasionally seen (WARD). 

 The greenish spots have been identified by Prof. BONNEY as pseudomorphs 

 after a rhombic pyroxene. They are generally a pale sap green by 

 transmitted light. Longitudinal sections show a faint pleochroism ; pale 

 green for rays vibrating parallel to the length of the section and yellowish 

 green for rays vibrating at right angles to this direction. They polarize in 

 weak tints, often showing the dark bluish tint of the first order, and always 

 give straight extinction. This mineral may be regarded as a variety of bastite. 

 It is interesting to observe that Avhereas the enstatite is almost always 

 altered, the augite is fresh. The enstatite, as Prof. BONNEY points out 

 must have largely exceeded the augite in amount. Many slides show no 

 augite in the form of porphyritic crystals. 



The above porphyritic constituents lie embedded in a greenish-blue or 

 dark slate-coloured matrix. Under the microscope this is resolved into 

 lath-shaped plagioclase, augite granules, magnetite and brown interstitial 

 matter. The interstitial matter is crowded with indistinct brownish 

 granules and small grains of magnetite. Sometimes it contains also 

 curved felspar microlites. It bears a close resemblance to the interstitial 

 matter of the andesitic-dolerites (augite -andesites) which occur as dykes in 

 the north of England (e.g. Cockfield dyke). Prof. ROSENBUSCH W refers to 

 a rock from Penrith which he states is essentially analogous to the 

 Labrador-porphyries of the Vosges. He classes it as a diabase-porphyrite. 

 The specimen referred to probably came from Eycott Hill. A more 

 definite idea of the character of the rock may be given by designating it a 

 labradorite-pyroxene-porphyrite. 



The lava flows which succeed the above are not characterized by 

 such conspicuous felspar-crystals. Some appear compact and non- 

 porphyritic. As a general rule, however, two generations of felspar may be 

 detected under the microscope. The ground-mass in all those which have 

 been examined consists of lath-shaped plagioclase, augite in minute 

 granules, magnetite and interstitial matter. The state of crystallization of 

 the ground-mass varies considerably. In some the constituents may be 

 recognized with a low power, in others they are so small as to require a 

 high power for their examination. The following analyses were made for 

 Mr. WARD by Mr. JOHN HUGHES (2) . 



(1) Massige Gesteine, 1st edition, p. 385. 

 (-2) Microscopic Journal, 1887, p. 24G. 



