54 Recent Excavations at Stonehenge. 
shows the same brecciform structure. From the analysis of Dr. 
Prevost of the stone 48 of the plan, and from the microscopic 
characters of all these rocks, it is evident that they represent old 
lavas of acid composition, which were either originally hornstone- 
like in structure (“hornsteimartig”) or more probably were more 
or less perfectly glassy, and have acquired their present hornstone- 
like lustre by secondary devitrification. Dr. Prevost’s analysis, 
showing an excess of soda over potash, points to the conclusion 
that they were rather quartz-andesites or dacites than rhyolites. 
Some of them may have been, as suggested by Professor Maskelyne, 
the consolidated and altered tuffs of such lavas. 
V. Sandstones, Grits, and Quartzites—The rock of the “altar- 
stone,” a micaceous sandstone, appears to be represented among 
the fragments dug up, and there are other more highly micaceous 
and very fissile varieties of sandstone among the fragments found. 
The rock of the altar-stone was thought by Professor John Phillips 
to resemble certain Devonian and Cambrian rocks; Professor 
Maskelyne pointed out its similarity to the well-known stone in 
the Coronation Chair, which, as Sir Andrew Ramsey showed, is 
almost certainly Old Red Sandstone from Perthshire; the late Mr. 
Thomas Davies states that a very similar rock of Devonian age is 
found outcropping at Frome, in Somerset. The more fissile and 
very micaceous varieties among the fragments are so similar to the 
Yorkshire flagstone, that there can be very little doubt of their 
derivation from rocks of Carboniferous age. With the sandstones 
are some quartzites, which do not appear to be indurated sarsen 
stones, but must be probably referred to true quartzites, like those 
of the Stiper Stones, Hartshill, etc. Altogether, among these 
siliceous rocks we appear to have possible representatives of various 
pre-Cambrian and Paleozoic sandstones. 
VI. Greywackés—TVhese are rocks consisting mainly of grains of 
quartz and of much altered felspar, with more or less argillaceous 
matter. They belong to the class of rocks so fully represented in 
the Scottish Borderland, and have evidently been derived from the 
- more or less imperfect disintegration of crystalline rocks. Although 
such rocks are especially abundant in the Scottish Borderland, 

