OF HERTFORDSHIRE BOULDERS. 
53 
apparently contains a few enclosures. A few black patches, prob¬ 
ably of magnetite, occur in places. 
Ashwell, 33 (PI. Y, f. 1).-—A rather coarse-grained granite, 
with an abundance of small silvery scales of muscovite. Felspar 
pink and rather kaolinised. A few black patches occur, which, 
when powdered, yield magnetic grains, and are probably magnetite. 
Under the microscope the rock appears as a light hrown, almost 
reticulated, mass of quartz and mica , with numerous dark fibrous 
streaks. The felspar is considerably decomposed, and no longer 
shows the characteristic structures with polarised light. 
Ashwell, 30 (PI. Y, f. 2).—Father coarse-grained admixture of 
orthoclase, quartz, muscovite, and a few black patches of biotite 
mingled with magnetite. The action of the magnet, together with 
blow-pipe tests, proves the comparative abundance of magnetite. 
Under the microscope the quartz shows many concentric rings of 
colour, with polarised light (owing probably to varying thickness 
of the granules towards the margins). It contains many cavities, 
both stone and fluid, but no needles of apatite were noticed. The 
felspar is rather turbid from decomposition, and is chiefly orthoclase; 
but one or two crystals show the characteristic twinning of plagio- 
clase. Muscovite appears to be abundant, and the black patches of 
magnetite are often associated with biotite , recognised by its strong 
dichroism. 
Sandon {from gravel-pit, near the Cat Bitch , 3) (PI. Y, f. 3).—A 
greenish-looking, hornblendic granite, with much pink orthoclase, 
medium-grained, and not much decomposed: appears rather gneissic, 
but the specimen is too small to judge with certainty. 
Under the microscope the felspar appears to be chiefly orthoclase; 
occasionally, however, there is a small patch of plagioclase . Quartz 
is not very abundant, and contains many stone and fluid cavities, 
sometimes of considerable size, but often so small as to appear, with 
•^in. objective, only as patches and lines of cloudy spots. Magnetite 
is abundantly present, often in well-formed cubical crystals. Both 
hornblende and biotite occur plentifully. Apatite , however, does 
not appear to be present. A noticeable feature of this rock is the 
tendency of the quartz to aggregate into lines, as well as the occur¬ 
rence of a few columnar, feebly-transparent crystals of an accessory 
mineral. 
5. Gneiss and Schists. 
Sandon {from gravel-pit , near the Cat Bitch, 4) (PI. Y, f. 4).— 
Yery small specimen of an apparently gneissose rock, containing 
decomposed orthoclase, quartz, and mica. Foliation is well dis¬ 
played by black bands, sparkling with mica, and yielding, when 
powdered, a considerable quantity of magnetite. 
Under the microscope the rock appears as a confused mass of 
felspar, mica (probably muscovite), and quartz, with faint signs 
of foliation. The whole mass appears brownish, and the felspar 
is much decomposed. A little biotite may be recognised. Magnetite 
forms dark streaks and black grains throughout the mass. The 
