Q2 E. H. L. SCHWARZ 
clase, hornblende, augite, quartz, red-brown mica, orthoclase, apatite iron ores, 
sphene and decomposition products such as chlorite, uralite and calcite. 
Variations in the proportions of these minerals show that the rock differs con- 
siderably in composition from point to point. 
The plagioclase has almost always a zonal structure; . . . . it frequently 
shows crystal outlines when in contact with the hornblende and augite, some- 
times small crystals of the feldspar are entirely inclosed by the hornblende and 
augite. This ophitic structure though found without difficulty in all slices of 
the rock examined, is not nearly so pronounced a feature as in the olivine- 
dolerite. 
The original hornblende is mostly of a pale greenish-brown colour, with 
feeble pleochroism, but a bright green strongly pleochroic variety also occurs, 
sometimes forming part of a crystal which is mostly made up of the pale kind. 
Occassionally small crystals showing the prism faces are met with, but the larger 
plates seen in the slices are always irregularly bounded by contact with other 
minerals, notably plagioclase. The last remark applies also to the augite, 
which is colourless in section and appears to be identical in character with the 
augite of the olivine-dolerite. The hornblende and augite usually occur 
together, intergrown with their orthopinacoidal faces parallel. The augite 
often forms the inner part of a section of the two minerals; outside this area 
the hornblende encloses the whole. The structure is easily seen by ordinary 
light under the microscope, as the augite is colourless and the hornblende pale 
greenish-brown, but between crossed incols the minerals are still more clearly 
seen owing to their extinguishing in different positions of the nicols. The 
intergrowths of the two minerals are sometimes twined, the composition plane 
being the ortho-pinacoid, common to both minerals. 
Hornblende is rarely found in the olivine-dolerites but it does occur in 
them, e.g., in the coarse olivine-dolerites of the sheet seen on the shore between 
the Gxacha and Kologha Rivers (Kentani) and in the Kologha sill. In a slice 
from the dolerite sill exposed along the Kei River at Mimosa Hill (the Kologha 
sill) there is much hornblende of the same variety as that in the Gap-rock and 
it is also intergrown with augite. 
The mica is a red, strongly pleochroic variety, frequently altered to a very 
pale, greenish mineral with weak double refraction. It frequently encloses 
small zircons round which there is always a pleochroic halo; zircon occurs 
similarly in the hornblende. This mica occurs frequently and is a most 
important constituent of the Gap dykes; a precisely similar variety is found 
in almost all slices of the Transkeian olivine-dolerites but in very small quantity. 
Quartz is abundant in some parts of the Gap dykes and present in all slides 
examined. It was the latest constituent to crystallise out from the liquid 
magma; it frequently forms a micropegmatitic intergrowth with a cloudy 
untwined feldspar which is probably orthoclase. Both micropegmatite and 
quartz are occasionally seen in the slices of the dolerites, but they are generally 
very subordinate constituents of the rock. 
