158 Stonehenge: the Petrology. of its Stones. 
from the breaking down and re-distribution of the pre-existing 
igneous rocks. 
The term schist as applied to the four Stonehenge “hornstones ” 
by Mr. Sowerby, is not erroneous. They have a marked schistoid 
structure, especially the stone 19, and this character is more obvious 
in some of the fragments, probably of other stones that have dis- 
appeared, found by Mr. Cunnington close by, and belonging to this 
type. 
These stones are harder than quartz. Their specific gravity varies 
from about 2.7 to 2.8, a specimen of the stone number 9 having the 
the specific gravity 2.783. 
It will thus be seen that they belong to a variety of sedimentary 
and somewhat metamorphosed rock which may be termed a quartzose 
felsite. 
An analysis of the stone No. 19, made by Dr. Prevost, gave the 
following as its composition :— 
Silica we ats 77.4 
Alumina ... ise 13.5 
Tron Oxide ib A 3.5 
Magnesia ... att 1.3 
Lime sie 333 1.8 
Potash nt At 0.6 
Soda a “ai 0.73 
Water a “ts 1.3 
100.13 
The following more particular description of each of the four 
stones will complete this notice :— 
No. 9. The ground mass of this stone is rich in grey chloritic mineral; it in 
fact might be looked on as having been originally a mud in which were mingled 
the fine quartzose sand and triturated crystals of felspar (not improbably oligo- 
clase) fractured or crushed by the forces that disintegrated the original rock from 
which the material wasderived, and in the metamorphosis of which the chlorite 
probably took itsorigin. The merely skeleton forms which often represent the fel- 
spar, shew that the process of metamorphosis continued after the deposit and 
previous to the solidifying of the materials of this chloritic paste; their shadowy 
forms being often entirely filled with the flocculent chlorite material, resembling 
