324 TJios. H. Waller — Volcanic Rocks, Nuneaton. 



The quartz grains, instead of having mostly rounded outlines, are in 

 very large proportion angular, and have a very fragmentary look. 

 The felspar grains, on the contrary, are very frequently rounded, and 

 the whole of the crystalline constituents are packed closely together ; 

 touching, in the majority of cases, as if they had been loose 

 grains. This peculiarity of aspect naturally varies in different 

 specimens, and in one or two I think I have detected a quartz grain 

 in the act of breaking up. It is of irregular shape, and at one side 

 is covered over with a network of strings of minute fluid cavities, 

 which divide it into roughly polygonal portions ; as we pass to the 

 other side of the crystal, however, these become lines of an infil- 

 trated green mineral, and at the extreme edge a few of the polygonal 

 fragments are quite detached, and separated from the main mass by 

 portions of the ill-defined ground-mass which occurs in this particular 

 specimen. Here the alternative lies between considering the fluid 

 cavities as the remaining indications of cracks which originally 

 divided the crystal, or looking upon the strings of cavities as original, 

 and as giving direction to the separation by being planes of weakness. 

 It is rather in favour of the -former assumption that the reticulated 

 arrangement of the cavities is, so far as my observation goes, more 

 frequent in the specimens which have the greatest appearance of 

 disturbance. Where the crystalline grains of quartz are indented 

 with intrusions of the ground-mass, there is very frequently a line of 

 cavities following the outline of the indentation. One specimen 

 shows very well the care needful in deciding that an apparent inclusion 

 is not in connection with the exterior. A quartz grain has a row of 

 four roundish apparent inclusions. The plane of the section, how- 

 ever, has fallen just within the neck connecting one of these included 

 masses with a little indentation of the margin, so that there is a very 

 faintly visible cloud observable showing the actual connection. 



In various places in this rock there are green patches and a few 

 black and brown ones. These are not so sharply defined micro- 

 scopically as they appear in hand-specimens. In some cases they 

 seem to have their margins indented with a quartz grain, or to 

 inclose a bit of felspar ; but occasionally they seem quite free from 

 any but microscopic crystals. The black patches, so far as I have 

 examined them, have the look of green serpentinous alteration 

 products, and some of the browner ones have much the appearance 

 of the neighbouring fine ashes, but others seem more filled with 

 minute lath-shaped crystals, which have much of the character of 

 felspar microliths. 



3. The Dark Basic Bock which is in contact with the quartz- 

 felsite in one little disused quarry has undergone great alteration — in 

 some parts to the complete obliteration of any original structure, in 

 others the components are very fairly preserved. Close to the 

 junction with the quartz-felsite, and running into it in strings and 

 tongues, the appearances strongly suggest a rock originally glassy, 

 which has been subsequently devitrified, like the glassy base of many 

 felsites. In some specimens the structure is markedly porphyritic ; 

 felspar crystals of considerable size occurring in a ground of crystals 



