Thos. H. Waller — Volcanic Rocks, Nuneaton. 325 



apparently of felspar of very small dimensions. In others the 

 crystals of felspar are larger and of more uniform size. Extremely 

 little augite has escaped, but it is curious that those which are pre- 

 served show, in addition to the twinning with the orthopinacoid as 

 the plane of composition, which is common, the unusual cleavage 

 parallel with the basal plane described by Mr. Teall as occurring in 

 the prevailing pyroxene of the Whin Sill of the North of England. 

 One section approximately in the plane of the clinopinacoid shows 

 the cleavages forming obtuse angles meeting in the plane of com- 

 position ; the angles were measured as 6Q° on one side, and 65° 

 on the other, and the extinctions 40° and 41° from the dividing line. 

 Where the colour of the augite can still be seen, it is very pale. 



The larger porpkyritically-developed felspars give extinction 

 angles such as indicate one of the more basic plagioclases, probably 

 labradorite. Some yellowish hornblende and a green chloritic or 

 serpentinous mineral are apparently products of alteration. 



The manner in which this rock, which seems to agree in characters 

 with Rosenbusch's diabase porphyrite, runs in among the quartz 

 and felspar grains of the quartz-felsite previously mentioned, the 

 presence of detached angular quartz grains in its mass, and the 

 appearance of flow which it has, lead me to believe that it flowed as 

 a lava over a broken disintegrated surface of quartz-felsite or of 

 a bed of sand derived from it. 



4. The Quartzite. — The quartz sand which has formed the founda- 

 tion of the quartzite shows in some of the lower beds its derivation 

 from such a rock as the quartz-felsite previously described, by the 

 survival of canals and inclosures of the felsitic ground-mass, and 

 indeed, in the conglomerate, quite at the base, little pebbles of felsite 

 occur along with angular fragments of the fine ashes. In the normal 

 quartzite the grains of primary quartz are well rounded, and contain 

 in many cases a great number of minute fluid cavities. The secondary 

 cementing quartz is much more free from cavities or inclusions 

 of any sort. It is an optical continuity with the primary quartz, 

 so that in polarized light the impression is given of polygonal areas 

 of colour. The appearance is markedly different where, as is quite 

 frequently the case, a grain of felspar, or of felsite, or of the ashes, 

 occurs. On these no quartz is deposited, but they are simply 

 enveloped in that which has crystallized on adjoining grains. 



5. The Diorites which occur in the quartzite are in an advanced 

 state of alteration, and often not much more than the shapes of the 

 old crystal forms can be made out. Calcite is developed in them to 

 a considerable extent. 



In a specimen from Tuttle Hill, collected by Mr. Teall, and kindly 

 lent to me by him, the hornblende is in the form of long and com- 

 paratively slender crystals, with frequent cross-divisions. A similar 

 structure occurs in a specimen from Merivale Church, kindly given 

 to me by Mr. W. J. Harrison. 



