152 Stonehenge: the Petrology of its Stones. 
employed. The other group, consisting of four stones, has been 
described under names which if somewhat less vague are not entirely 
incorrect, namely, “hornstone and siliceous schist ” (Sowerby), or 
also “compact felspar of Mac Culloch” (Phillips). The stones in 
question are 33 in number. Sir R. Colt Hoare enumerated 31, all 
of which ean be recognised, but the stone marked 6 in his enumera- 
tion is now in two fragments, and a small outlying stone in a line 
with the outermost sarsen circle belongs also to this elass. It lies 
near that numbered 5 on his plan. 
Of these stones the four numbered 9, 11, 17, and 19, belong to 
the class named by the late Mr. Sowerby, siliceous schist (No. 9), 
and hornstone with specks of felspar and pyrites (Nos. 11, 17 and 
19). The remainder of the stones are those which have been termed 
greenstone and syenite. Greenstone is a vague term which might 
include diabase; but qualified by ‘the reference to syenite, it is in- 
correct as a description of these Stonehenge stones. In fact Professor 
Phillips and Mr. Sowerby appear to have mistaken the augite con- 
tained in these so-called greenstones for hornblende. Such an error 
however might most easily be fallen into at a time when it was far 
less usual than now to have recourse to the aid of the microscope, 
and of thin sections of the rocks cut for use with that instrument. 
Modern petrology, by employing this method of investigation, and 
by further scrutinising the inmost characters of a crystallised sub- 
stance by polarised light, is able to pronounce, often with a precision 
previously impossible, upon the true character of a mineral constituent 
of a rock. Thus examined by the aid of the microscope, the Stone- 
henge stones, of the so-called greenstone type, are seen for the most 
part to consist of a mass of somewhat opaque white or greenish- 
white felspar; commingled with which are broad crystals of a brown 
and sometimes greenish augite. The felspar can be recognised as 
one of the anorthie kinds; probably it is labradorite, but the con- 
dition of the mineral renders it very difficult to settle this point, 
even by analysis; for it is considerably decomposed. Internally it 
is seen to be thickly clouded with a flocculent mineral of a grey or 
greenish-gray, and sometimes distinct green colour : a mineral which 
is readily recognised as chlorite or one of the varieties of mineral 
