640 8. ALLPORT ON THE DIORITES OF 
crystals with a few of orthoclase ; among these are scattered nume- 
rous small crystals of brown hornblende, many crystals and grains 
of clear yellowish augite, many grains of magnetite, a few needles 
of apatite, and several pseudomorphs after olivine. There is also a 
matrix in which the original constituents are set, but whether glassy 
or felsitic in its original state, there is no evidence to show; it has 
undergone a considerable amount of alteration, and now consists 
generally of a pale green serpentinous substance frequently accom- 
panied by calcite. In several instances the usual forms of the augite 
crystals are perfectly well seen, some of them being twins; they ex- 
hibit no trace of dichroism when examined without the analyzer in 
the same way as the hornblende ; and as well-formed crystals of both 
minerals occur in a single slice, their crystallographic and optical 
characters may be observed side by side. The augite appears to be 
very irregularly distributed throughout the mass of the rock; for in 
some slices it is almost as abundant as the hornblende, while in 
others it is nearly or even quite absent. It is slightly altered here 
and there, but never resembles uralite, the altered parts having 
merely a turbid grey aspect, like that observed in many of the 
dolerites. 
Having given special attention to the various kinds of pseudo- 
morphs after olivine, I had no difficulty in detecting their presence 
in several of these rocks; the discovery was, however, so entirely 
unexpected that, after cutting many slices, it was no small satisfac- 
tion to meet with a thoroughly characteristic form of the crystal. 
Some of these pseudomorphs consist exclusively of calcite, while in 
others the central parts are filled with viridite; they thus correspond 
in every way with many observed in the more highly altered dole- 
rites described on a former occasion*. In some slices the pseudo- 
morphs are very numerous and are generally larger than the crystals 
of augite or hornblende; they nearly all contain a few grains of 
magnetite, but never any other of the original constituents. The 
fissured condition of the unaltered crystals is also clearly indicated 
by the veins so familiar to those who have studied this mineral in 
the older dolerites. These and other features described in the paper 
just referred to are so thoroughly characteristic, that there is no 
room for doubt as to the former presence of olivine in these rocks. 
There is also present in some quantity another mineral of which 
it will be well to speak less positively. It is quite colourless and 
occasionally clear, though it generally contains more or less of a fine 
dust which gives it a cloudy appearance, especially round the mar- 
gins. Some of the sections are hexagonal, others rectangular; the 
former are dark between crossed Nicols, while the latter are 
coloured, though the tints are not brilliant. The crystals certainly 
belong to the hexagonal system, and are larger and stouter than the 
ordinary needles of apatite, many of which also occur in these rocks ; 
it is possible, I think, that. they may be nepheline. Against this 
if ie Allport, “On Carboniferous Dolerites,” Quart. Journ. Geol. Soc. vol. xxx. 
p. 042, 
