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 (" hornsteinartig ") 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 Eamsey 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 Pakeozoic sandstones. 



VI. GreywacMs. — These 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, 



