J. J. Harris Teall—Cheviot Andesites and Porphyrites. 105 
the ground-mass are more acid than those which occur as large 
crystals, and which certainly belong to an earlier stage in the pro- 
cess of rock consolidation.!. In the Santorin lava of 1866, the por- 
pbyritic felspars consist of labrador, anorthite, sanidine and oligo- 
clase, and the felspars of the ground-mass of albite and oligoclase, 
according to M. Fouqué.? Another constituent of the ground-mass 
is a well-crystallized mineral which occurs in extremely thin hexa- 
 gonal plates of a deep brown colour. The largest of these plates 
measure only about 03 mm. across, and therefore they may be easily 
overlooked when low powers only are used. It is doubtless either 
hematite or biotite, and on account of its great absorbing power in 
very thin plates, I am inclined to regard it as the former. 
Colourless acicular microlites, brownish granules (globulites), 
and black opaque grains (magnetite), together with a base of true 
isotropic glass, sometimes brownish and sometimes colourless, make 
up the remaining portion of the ground-mass. I should have 
mentioned before that colourless prisms of apatite occur somewhat 
sparingly. The different constituents of the rock have consolidated 
in the following order: magnetite, pyroxene, porphyritic felspars, 
felspars of the ground-mass, base with devitrification products. 
Secondary products.—The most striking of these are connected 
with the red veins, which frequently traverse the rock in different 
directions. The thin veins present, under the microscope, a deep red 
homogeneous tint. The thicker ones, however, are usually separated 
by a narrow transparent band of clear quartz or chalcedony. 
Scattered here and there throughout the clear substance are extremely 
minute particles of ferrite, and these are occasionally seen collected 
into more or jess spherical masses, which remind one, in their mode 
of aggregation, of Vogelsang’s cumulites.? Sharply-defined rays may 
be seen extending outwards from these spherical masses, although 
nothing like a radial structure can be observed in the masses them- 
selves. Agegregations of the spherical masses may sometimes be 
seen shading off into the homogeneous red substance (jasper) which 
forms the margin of the vein. The red bands are separated from 
the clear quartz by a wavy line, which indicates that the interior 
surface of the jaspery portion of the vein is mammillated. The red 
colouring matter is not only collected in the vein itself, but may 
frequently be seen extending into the mass of the rocks for some 
little distance on either side. 
In addition to the veins there occur, very sparingly in the andesites, 
though abundantly in the ordinary porphyrites, drusy cavities lined 
with a coating of chalcedony, and more or less filled with a clear 
green substance, sometimes appearing isotropic, and sometimes giving 
a feeble aggregate polarization. 
Structural variations.—Only two important types of structure have 
been observed; in the one, the more common type, the porphyritic 
1 Rosenbusch, H., Ueber das Wesen der kérnigen und porphyrische Structur bei 
Massengesteinen, Neues Jahrbuch, 1882, p. 13. 
+ Santorin et ses Eruptions, p. xi. 
3 Die Krystalliten, Bonn, 1875, page 139. 
