By Nevil Story Maskelyne, u.A., PRS. 153 
Spawn under the chlorite group. Sometimes this mineral is seen 
in distinct triangular plates of a bright green colour; and erystalline 
lamine of it are enclosed occasionally in the augite, sometimes even 
in such a way as suggest a doubt whether the chlorite is not con- 
temporaneous with the rock, and not, in every case at least, a product 
of its decomposition. The augite presents less decomposition than 
the felspar: the crystals are broad in proportion to their length, and 
give a sort of micro-porphyritic character to the rock. Magnetite or 
Ilmenite, it is difficult to decide which (probably the former), is an 
occasional ingredient in minute crystals; and quartz is present, 
though very unequally distributed in small granular, but, under the 
microscope, recognisable crystals. Of ground-mass strictly speaking, 
that is to say as distinct from the closely-packed crystals of felspar, 
there is little to be recognised in most of the sections that I have 
made, though in one or two it is more abundant. Where it can be 
recognised it consists of crystalline felspathic grains, that seem less 
decomposed than those forming the crystallised mass, and may 
possibly belong toa different felspar. Occasionally augite is seen 
mingled with these, or a mineral which appears to be augite as ex- 
amined in polarised light, and it also occurs there in minute granular 
crystals. 
From this description it is clear that the igneous rock, to which 
the obelisks of this class must be ascribed, can only be called a 
diabase. But for its chloritic ingredient it might be termed a 
dolerite. But whether the difference between a diabase and dolerite 
be, as is more probable, simply one of longer exposure, and, as a 
consequence, a more complete succumbing on the part of the diabase 
to the disintegrating influences of metamorphic action; or, whether, 
on the other hand, we are to recognise in them two similar rocks 
which have taken the solid form under different conditions of pressure 
and of thermal action; in either case the chlorite, and I may add, 
a small amount of calcium carbonate, detected by acid definitely 
place these Stonehenge obelisks in what is now recognised as 
the diabase division of the old class of rocks that have been known 
by the vague name of greenstone. 
An analysis of a fragment from the stone No. 4 was made 
