154 Stonehenge: the Petrology of its Stones. 
made by Dr. Prevost in my laboratory at Oxford, with the following 
results :— 
Silica ee ¥F 51.7 
Alumina ... iss 12.1 
Tron Oxide 24% aS 15.3 
Magnesia ... sae 4.08 
Lime nee a 10. 
Potash aig ff 1.02 
Soda +e ~ 2.8 
Water fe eed 2.6 
99.6 
The iron was estimated as ferrie-oxide, but a portion of it was 
probably present in the ferrous condition. 
The following is a review of the particular characters of the 
stones which belong to this diabase division : the numbers indicating 
the different stones being those by which Sir R. Colt Hoare desig- 
nated them in his plan and description of Stonehenge :— 
Nos. 1 and 3 are diabase of average grain, the augite and the felspar being 
very visible on the surface weathering out from the stone. They are quite 
similar and might have been parts of a single block. Some hexagonal crystals 
in No. 8, when examined by polarised light, present the characters of quartz. 
No. 2 is the prostrate lintel, curved in form and presenting two worked de- 
pressions for the reeeption uo doubt of protruberances on the upright stones 
that supported it. The grain is coarser than that of fhe two former stones, 
The augite is present in considerable quantity in the form of short broad 
erystals of a greenish-brown colour. The augite sometimes contains crystalline 
chlorite and it aids by the size of its crystals in giving the rock a porphyritic 
character, which is also greatly due to the felspar crystals which are often 
distinct. The latter are of a greenish hue in places where the felspar has sur- 
vived decomposing influences; but in the greater part of its mass this mineral 
has become metamorphosed into a fleshy white nearly opaque mass, retaining 
only the form of the original crystal; and this mass is full of the flocculent 
chlorite that has been described, and which is in fact crystalline though only 
recognisable as such by high powers of the microscope. Magnetite (or 
Ilmenite) is also present in small amount and a little quartz in small granular 
crystals, 
No. 4 The structure of this stone though porphyritic is more compact and 
finer-grained than that of the last stone. ‘The felspar is, in some parts, less 
decomposed than is the case in No, 2. The felspar of the stune No. 2 has 
