By William Gowland, F.S.A., F.I.C. 



53 



rock of this type is so great that it is probable a number of stones 

 of this kind were used in building Stonehenge. Owing, however, 

 to their fissile character, and the ease with which they succumb to 

 weathering agencies, all seem to have disappeared except a stump, 

 which was discovered by Mr. H. Cunnington in 1881, 1 and referred 

 to by Mr. W. Cunnington as S 28 of his plan. 



IV. Altered Rhyolites and Dacites. — These rocks have been 

 variously referred to by previous authors as " hornstones," " por- 

 phyrites," and " felsites." They are probably the compact felspar 

 of Phillips. One striking porphyritic variety has often been 

 referred to under the name of " black quartzite with felspar 

 crystals." It has occurred not only among the fragments which 

 were obtained by Mr. Gowland, but also in the series of specimens 

 collected from the surface by Mr. Cunnington, while one large 

 piece was found in one of the barrows (No. 41, Hoare's map). It 

 is probable that a stone consisting of this material once existed at 

 Stonehenge, but has now disappeared. The striking appearance of 

 the rock would probably lead to " specimens " being frequently 

 knocked off from it, and thus, in the course of years, the whole 

 stone may have disappeared. 



All of the rocks of this class show very strikingly the " fluxion 

 structure," and there is no doubt that they represent old lavas of 

 a viscid character, in which the material has been drawn out in 

 the direction of the flow. Professor Maskelyne has remarked upon 

 the existence of clastic structure in the stones 38, 40, 46, and 48 

 of the plan. This would seem to indicate that these stones represent 

 not lavas but tuffs or agglomerates composed of lava-fragments. 

 But upon this point I would suggest that a brecciated appearance 

 is often exhibited by true lavas, like the well-known " piperno " of 

 the Pianura near Naples. Indeed the name of " eutaxites " of 

 Fritsch and Eeiss, and the term " eutaxitic structure," are recog- 

 nition by petrographers of the fact of the existence of such 

 " brechoiclal " lavas. Mr. Eutley has shown that the celebrated 

 porphyry of Djebel Dokhan, the original prqfido rosso antico, often 



1 Wilts Arch. Mag., xxi. 142. 



