496 1 PROFESSOR GEIKIE ON THE 
above described as occurring in the diabases. The triclinic felspar, though 
often singularly unchanged, may be detected passing into the granular kaolinised 
condition. The augite, as in the diabases, remains on the whole less affected 
than the felspar. It may, however, be observed with a surrounding border 
sometimes of an opaque black, sometimes of a dirty brown substance, which 
graduates inward into the still fresh augite. In other instances the external 
decomposed coating consists of serpentine, and strings of the same mineral may 
be traced along the fissures of the augite. The decomposition into serpentine 
may also be observed to have occasionally begun in the centre, while the 
surrounding part of the augite still remains tolerably fresh. Much diffused green 
matter, the so-called ‘viridite,’ may represent both olivine and augite. 
Leucoxene occurs under the same conditions as in the diabases. Hematite 
and limonite have often replaced the original iron oxides. Pyrite is almost 
always present in minute grains. Calcite, various zeolites, quartz, and 
calcedony fill up the pores and cavities of the rocks, and often run in 
veins through them. 
Phenomena of Contact.—It is well known that an intrusive crystalline rock 
assumes a Close texture along its junction with the rocks through which it has 
been thrust, and that these in turn commonly show more or less induration. I 
have examined a large number of microscopic sections taken both from the 
igneous and aqueous rocks, and will state here the general results of the 
investigation. 
Tracing the variations of an intrusive dolerite outwards in the direction of 
the rocks which it has invaded, we perceive change first in the augite. The 
large crystals and kernels of that mineral grow smaller until they pass into a 
granulated form like that characteristic of basalts. The large plates and amor- 
phous patches of titaniferous iron or magnetite give place to minute particles, 
which tend to group themselves into long club-shaped bodies. The labradorite 
continues but little affected, except that its prisms, though as defined, are not — 
quite so large. The interstitial glassy ground-mass remains in much the same 
condition and relative amount as in the centre of the rock. In figs. 11 and 12 
of Plate XII., I have drawn the structure of two interesting examples of contact 
phenomena. 
Along the line of contact with a sandstone or other granular rock, the 
dolerite becomes exceedingly close-grained. Its felspar crystals are still quite 
distinct even up to the edge of the stratified rock, but are fewer in relative — 
number, and still smaller in size, though an occasional prism two or three — 
millimetres in length may occur. They retain also their sharpness of outline, 
and their comparative freedom from enclosures of any kind. They tend to 
range themselves parallel with the surface of the sandstone. The augite exists 
as a finely granular pale green substance, which might at first be taken for a 
