THE QUIZZYHOTA LACCOLITE 93 
The iron-ores are magnetite, ilmenite with which sphene is often associated 
and iron pyrites. Apatite is always present, sometimes in considerable 
quantity. 
The rock forming the dykes of the Gap may be called quartz-mica-augite 
diorite; it differs from the olivine-dolerite very considerably in the absence 
of olivine and in the presence of large amounts of hornblende, brown mica and 
quartz as well as the more acid varieties of plagioclase. It is very noticeable, 
however, that none of the minerals which characterise the Gap-rock are foreign 
to the olivine-dolerites and in the case of the Kologha sill the dolerite in parts 
approaches the Gap-rock in character rather closely by the increase in the 
amount of hornblende, red mica and the zoning of the plagioclase. The 
affinity between the two rocks is sufficient to make it preferable to regard the 
Gap-rock as derived from the magma which supplied the dolerite intrusions 
rather than the result of a quite different order of events. If we consider the 
Gap-rock as a late product of the magma after the dolerite had been got rid of, 
our view will explain the facts observed under the microscope and in the field; 
for while the evidence of a microscopic examination shows that the Gap-rock 
and the dolerite are genetically related, the field evidence conclusively proves 
that the latter rock had solidified before the former was intruded through it. 
I do not think the last-mentioned fact invalidates the present 
view that these diorite dykes are the channels of escape of waste 
siliceous material as well as supply dykes of basic material. The 
dolerite spreads upward and the conduits of the upper sills must 
have remained open after the consolidation of the lower sills. There 
has been noted also a sequence in the infillings of the composite 
dykes of Arran and Skye. It may be noted that in the latter 
locality, where the granite invades the gabbro, it often partially 
fuses it and converts it into a rock consisting of hornblende and 
feldspar (labradorite to oligoclase).‘ At any rate the absorption 
of the sediments by the dolerite necessitates the escape of siliceous 
material, and the diorite dykes, where the differentiation into an 
acid and basic series has not been completed, are of the necessary 
constitution. The fact that the diorite dykes have been noticed 
where there is great development of laccolites, as at Cradock and 
in Komgha, is of special importance. The ordinary sills of the 
Karroo have been intruded for miles along inclined planes and, as 
the thickness of the dykes and sills varies very little, the diffusion 
of the acid and basic parts of the magma would have been unre- 
strained. In the laccolities, however, which form great lumps with 
1 A. Harker, Tertiary Igneous Rocks of Skye, 171- 
