50 
J. Y. ELSDEX—MICROSCOPIC STRUCTURE 
Under the microscope there is an abundance of moderately fresh¬ 
looking plagioclase , enclosing crystals of apatite. The augite is 
brown in colour, and there are a few minute patches of a brown 
strongly dichroic substance, probably biotite. Olivine is not very 
abundant, and is altered completely. Magnetite is present as usual, 
and there is a considerable quantity of brown and yellow products 
of decomposition. 
Ashwell, 51 (PL III, f. 3).—Black, crystalline rock, apparently 
fresh and undecomposed. 
Beneath the microscope olivine appears to be tolerably abundant 
in semi-porphyritic crystals, which are seen in every stage of 
alteration, but the majority are clear and transparent, except along 
the cracks. The products of alteration appear to be the usual 
fibrous serpentinous substance, and a little hematite and magnetite.' 
One crystal has enclosures of magnetite grains, arranged symmetri¬ 
cally around the margin, somewhat in the manner so characteristic 
of leucite and nepheline. Felspar is abundant, and apparently 
wholly triclinic: it is fairly unaltered, polarises brilliantly, and 
sometimes contains apatite. It occurs not only in distinct prismatic 
crystals, but also in crypto-crystalline masses, and is sometimes 
penetrated by a fibrous, interlacing system of lines showing in¬ 
cipient alteration. Augite occurs sometimes in well-formed crystals, 
sometimes in irregular patches. Magnetite is abundant, both in 
square sections, and in irregular patches. 
2. Basalts. 
Ashwell, 124 (PI. Ill, f. 4).—A dark compact basalt, apparently 
rather decomposed. This is a very small specimen. 
Under the microscope abundant crystals of plagioclase appear 
disseminated through a dark-brown granular-looking matrix, which 
is too opaque for any other crystals to be distinguished. The felspar 
encloses magnetite and frequently shows a brownish-yellow decom¬ 
position-product. Magnetite is abundant throughout, but neither 
augite nor olivine can be distinguished in the matrix. 
Ashwell, 11 (PI. Ill, f. 5).—A compact, bluish-black basalt, 
with semi-porphyritic crystals of olivine. This specimen looks 
fairly fresh. 
Under the microscope there is seen to be a great deal of olivine , 
in comparatively large crystals, and many not much altered. 
Parts of the crystals are often converted into a fibrous, serpentinous 
or chloritic substance, and included crystals of magnetite are 
common. Some complete pseudomorphs of serpentine or chlorite 
occur. Magnetite is abundant, generally in well-marked cubical 
crystals, and a few rhombohedral sections are probably titaniferous 
iron. Felspar is abundant in rather small prismatic crystals, 
which, when they polarise well, show the characteristic striae of 
plagioclase, but they are often so imbedded and mixed up with the 
ground-mass as to be scarcely recognisable. Augite often occurs in 
well-defined isolated crystals, possessing faint dichroism (purple- 
brown to brown), and enclosing grains of magnetite. A true glassy 
