. Geological magazine. f long ellipsoidal joint-surfaces. These at one placeare crossed and partly obliterated by a set of wavy joints, so thatthe rock is cut up into small rhomboidal pieces. Where the fissilestructure is best developed, there the cross-joints are least. Towardsthe eastern end the divisional planes of the fissile structure curveround, and really form parts of flattened spheroidal surfaces, and inso doing cut the banded structure at high angles (Fig. 1). 1 Mentioned by Bryce, Geology of Arran, p. 72. ^ The base of the felsite near the basalt has a vitrified appearance and has a
. Geological magazine. f long ellipsoidal joint-surfaces. These at one placeare crossed and partly obliterated by a set of wavy joints, so thatthe rock is cut up into small rhomboidal pieces. Where the fissilestructure is best developed, there the cross-joints are least. Towardsthe eastern end the divisional planes of the fissile structure curveround, and really form parts of flattened spheroidal surfaces, and inso doing cut the banded structure at high angles (Fig. 1). 1 Mentioned by Bryce, Geology of Arran, p. 72. ^ The base of the felsite near the basalt has a vitrified appearance and has a faintindication of banding. Rev. T. O. Bonney—Pitchstones and Felsites of Arran. 501 Pitclistones, Tormore sliore.—The soutliernmost of these dykesexhibits banded, laminated and rudely columnar structure. Thefirst is generally parallel to the upper and lower surface; so also isthe second, but not invariably, as the two structures do not alwayscorrespond in direction, and occasionally cut at an angle ofnearly 80°.. Fig. 1.—Diagi-am of intersection of streaky and laminated structure, from LowerPitchstone, Dunfion. A. Laminated structure, rather curved. B. Streaky structure. A dyke of brown pitchstone to the north of the last, about fourfeet wide, shows banded structure very clearly on the southern side,where there is an intruded basalt dyke. This structure is barely, ifat all visible on the northern. The above cases seem to show that, though the fissile andbanded structures are often coincident by reason of a common re-lation (which will be presently discussed) to the surface of the mass,yet they are of independent origin ; and that the former is reallyonly one special case of a series of phenomena due to contractionof the rock in cooling. Many other instances showing the relationof fissile structure to tabular and columnar jointing, and thatbetween these and spheroidal structure, might readily be adducedfrom the various Arran rocks; but it is perhaps needless to multiply
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Keywords: ., bookauthorwoodward, bookcentury1800, bookdecade1870, bookyear1877
