OF HERTFORDSHIRE BOTJLDERS. 
51 
matrix is undoubtedly present in considerable quantity, and 
abundant microlitbs are visible with a higher power. A good rock 
for the study of olivine pseudomorphs. 
Ashwell, 25 (PL III, f. 6).—A. dark, moderately fine-grained 
crystalline rock, decomposing on the outside into an earthy mass, 
with much red oxide-of-iron. Olivine is abundantly visible in 
greenish masses, and moderately large crystals of pyroxene are 
plentiful. 
Under the microscope the most abundant constituent is seen to 
be olivine, which occurs both in irregular grains and in large semi- 
porphyritic crystals. As a rule it is not much altered except on 
the margins, and along the numerous cracks. The alteration- 
product is a fibrous, green substance resembling serpentine. It 
frequently encloses magnetite. Felspar is not in great abundance, 
and consists chiefly of plagioclase, frequently enclosing apatite. 
The pyroxenic element is augite , occurring in large brown patches 
often showing feeble diehroism (purple to brown). Patches of 
magnetite are common, generally in square sections. Magnificent 
olivine pseudomorphs occur also in this rock. 
Ashwell, 125 (PL IY, f. 1).—A compact, dark basalt, of a 
greenish-grey colour. Small particles of pyrites are the only 
ingredients distinguishable. 
Under the microscope the rock appears as a brownish, glassy 
matrix, in which are scattered grains of nearly unaltered olivine. 
Augite is also present in great abundance, in very minute crystals. 
Felspar appears to be very scarce, but grains and crystals of 
magnetite and pyrites are abundant. 
Ashwell, 27 (Pl. IY, f. 2).—A black, compact basalt, showing 
green grains of olivine, and a few amygdaloidal cavities filled with 
calcite. A fairly fresh-looking specimen. 
Beneath the microscope abundant crystals of plagioclastic felspar 
are seen in rather small crystals. Olivine is plentiful in rather 
large crystals and grains, and almost completely altered to a fibrous 
green serpentinous mineral. Magnetite is very abundant, showing 
chiefly square sections, but a few have the appearance of titaniferous 
iron. Augite appears to be much altered to a brownish, granular 
substance, and in some cases has completely disappeared, leaving 
only skeleton-crystals of black magnetite to mark its former exist¬ 
ence. An abundance of microliths is to be observed in the glassy 
matrix. As secondary products, serpentine , calcite , and red patches 
of ferric-oxide occur. 
3. PoRPHYRITES. 
Ashwell, 66 (Pl. IY, f. 3).—A, purple, compact matrix, with 
felspar porphyritically imbedded, in tolerably large crystals. Speci¬ 
men very small, and exposing no very fresh fracture. Much 
tarnished with red oxide-of-iron. 
Under the microscope large crystals of triclinic felspar are seen, 
showing the characteristic twinning in a very marked degree, and 
containing numerous needles of apatite , greenish-yellow chloritie 
