52 
J. Y. ELSDEN—MICROSCOPIC STRUCTURE 
matter, and also portions of the granular base. A considerable 
quantity of the usual turbid alteration-product is present in some 
of the crystals. The pyroxene is completely altered to a greenish- 
yellow fibrous substance, in many cases bounded by the usual 
outlines of the crystals. Magnetite is rather sparingly present. 
The matrix is brown and granular, and appears to contain true 
glassy matter. Numerous minute veins ramify through it, infiltrated 
either with a brownish-red opaque substance (? hematite), or with 
a transparent doubly-refracting substance, probably calcite. The 
brown matter frequently has the appearance of broken skeletons, 
representing the fragmentary outlines of what may once have been 
crystals. 
Ashwell, 141 (PI. IY, f. 4).—A very small specimen, showing 
porpbyritic crystals of felspar, imbedded in a purple, compact 
matrix. 
Under the microscope the felspar is seen to consist of large 
crystals of plagioclase, and probably some orthoclase, although in 
many cases it is too much decomposed to give the usual optical 
characteristics. It is often greenish, in places, from admixture 
with chloritic matter and portions of the base. Pyroxene is only 
represented by a number of greenish pseudomorphs ( viridite ) some¬ 
times surrounded by a ring of segregated iron. Magnetite is plentiful, 
and in part appears altered to hematite , which appears as red plates 
occasionally. The matrix is a greenish-grey, compact, granular 
mass, consisting of glassy matter, small crystals of felspar, altered 
pyroxene, and granules of magnetite. Apatite occurs sparingly both 
in the felspar and in the ground-mass. 
Ashwell, 122 (PL IY, f. 5).—A compact, greenish, somewhat 
serpentinous-looking rock. With a pocket-lens minute lustrous 
specks are visible, having the appearance of mica. 
Under the microscope shadowy porphyritic crystals of felspar 
appear in abundance; hut they are so full of the ground-mass that 
they are difficult to identify optically. Some are undoubtedly 
triclinic. Magnetite is sparingly present, and mica is represented 
by biotite. Chlorite appears to be abundant in green scales, scattered 
through the base, and associated with it are small radiating nests of 
a strongly dichroic mineral, having the appearance of actinolite. 
The base is crowded with minute, transparent granules, which 
polarise brilliantly and are possibly silica. It is difficult to say 
whether any true glass is present or not. 
4. Granites. 
Ashwell, 72 (PI. IY, f. 6).—A coarse-grained, grey granite: 
felspar apparently rather kaolinised. 
Under the microscope the felspar appears almost entirely con¬ 
verted into an opaque cloudy mass, hut showing occasionally, with 
polarised light, the characteristic appearance of orthoclase. Quart% 
occurs in moderately large masses, and containing many enclosures 
of apatite as well as stone and fluid cavities, the latter with the 
characteristic bubble. The mica appears to he wholly biotite, and 
