79 



Chips of Stones Classified. Stonehenge. 



I. — Sarsens. They are concreted portions of a tertiary sand- 



stone, either of the Woolwich and Reading beds, Prest- 

 wich, or of the Bagshot beds. Micro-sections show that 

 in some cases they are coarse grained, in others very fine 

 grained ; the sand grains of which they are composed are 

 sometimes well rounded and at other times angular. 

 Other minerals are sometimes present — felspars, mica, 

 glauconite and chips of flint. The grains sometimes show 

 only a small quantity of cement between them — at other 

 times this silicious cement is in large quantity, and the 

 outlines of the original grains can be traced only with 

 difficulty, the rock being almost undistinguishable from 

 a quartzite. 



II. — Diabase (greenstones of hornblende and felspar). Some 



are coarse grained and porphyritic, others are fine grained 

 and almost compact. 



III. — Highly altered basic tuffs and agglomerates, generally of a 



fissile character. They have been called " calcareous 

 chloritic schists," and are evidently of volcanic origin. 

 Fragments of such rocks are so numerous that probably 

 many stones of this kind were used in building Stonehenge. 

 But owing to their fissile character and the ease with 

 which they succumb to weathering all seem to have dis- 

 appeared except a stump which was discovered by H. 

 Cunnington in 1881. 



IV. — Altered rhyolites and dacites, often called hornstones. 



It is probable that a stone consisting of this material once 

 stood at Stonehenge, but has now disappeared. They 

 show fluxion structure, and represent old lavas. 



V. — Sandstones, grits and quartzites. The rock of the " altar 



stone " is a micaceous sandstone. A similar rock is found 

 outcropping at Frome, in Somerset. The quartzites do 

 not appear to be indurated Sarsens, but may belong, like 

 the Stiper-stones, to true quartzites. (42). All these rocks 

 are possibly representatives of pre- Cambrian and palaeozoic 

 sandstones. 



VI. — Greywackes — which consist mainly of grains of quartz, 



much altered felspar, and argillaceous matter. Such stones 

 exist in Wales, Devon and Cornwall. Their absence from 

 the existing circles at Stonehenge is doubtless due to their 

 fissility and ease of decomposition. 



VII. — Argillaceous flagstones and slates. These would be 

 especially liable to break up by frost, &c. 



VIII. — Glauconitic sandstone. 



IX. — Flints. These are probably fragments struck off in 



renewing the points of tools (and perhaps in working other 

 stones). 



Most of the rock fragments are perfectly angular. 



42 — The Stiper-stones, it may be said, are silicious sandstones which occasionally- 

 pass into crystalline quartz-rock. — Horace Woodward. 



