152 J. J. Harris Teall—Cheviot Andesites and Porphyrites. 
containing altered felspars and a dark greenish mineral derived from 
the pyroxene. 
Under the microscope it is at once seen to be most closely allied 
to the pitchstone-porphyrite which occurs in the immediate neigh- 
bourhood. It is a rock of essentially the same composition and 
structure ; the same constituents may be recognized down even to the 
minute reddish-brown mineral which occurs in thin hexagonal plates 
(? hematite) in the ground-mass. There was doubtless some slight 
physical difference in the condition of the ultimate base of this rock 
which led to its being more easily attacked by the ordinary agents of 
weathering than its glassy relative; such a difference, however, may 
have been produced by slight differences in the conditions under 
which the rock consolidated, so that after all this porphyrite and the 
andesite with a glassy base may have originally been portions of one 
and the same molten mass. As there is such a marked macroscopic 
difference in the two kinds of rock I did not suspect their close 
relationship in the field, and consequently did not look closely for 
evidence as to their connexion with each other. 
In the thin section the felspars are seen to have been slightly 
attacked and in the usual way ; iron oxides are scattered pretty 
abundantly throughout the rock, and some of these appear to be 
of secondary origin. The most interesting mineral, however, is 
the one which exhibits the characteristic form of the rhombic 
pyroxene. It is now green in all sections, but still pleochroic in 
various shades of green. Cross-sections have lost their prismatic 
cleavages, and so also have the longitudinal sections; these latter, 
however, sometimes contain numerous rigidly parallel interpositions 
of perfectly straight hair-like rods, which were probably present in 
the original mineral. When examined under a high power, the 
substance is seen to be not perfectly homogeneous, and longitudinal 
sections show a somewhat indistinct parallel fibrous structure. At 
times very minute fibres may be seen to proceed from a central 
axis, exactly like the barbs of a feather. Under crossed Nicols the 
mineral approximates in character to an orthorhombic crystal, but 
owing to the want of uniformity above referred to, the sections do 
not give perfectly definite extinctions. We have here, doubtless, a 
mineral closely allied to bastite, and like bastite’ occasionally 
passing, by almost imperceptible gradations, into serpentine. 
EXPLANATION OF PLATE IV. 
Figure I. is described in the text. 
Figure II. represents a thin section of the prevailing type of Cheviot andesite, 
described in the last number of the GEOLOGICAL MAGAZINE. The twokinds of felspar 
are well shown; the earlier and larger crystals with inclusions, and the later and 
smaller ones for the most part without inclusions. A longitudinal section of the 
rhombic pyroxene occurs immediately above the large porphyritic felspar. The 
microscopic ground-mass is not well represented in the Figure; it consists of black 
opaque grains (? magnetite), a minute hexagonal mineral occurring in thin plates 
(? hematite), and a considerable amount of nearly colourless isotropic glass. 
1 Levy and Fouqué, Min. Micro. p. 383. 
(To be continued.) 
Errata IN Previous Paper.—Page 103, line 47, for 0’ to 44’ read 0° to 44°; 
page 108, line 4, omit the word not; page 108, line 23, for Nape read Nahe. 
