




























By William Gowland, F.S.A., FIC. 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,' 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 
erystals.” It has oceurred 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 
ol [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 Reiss, and the term “eutaxitic structure,” are recog- 
nition by petrographers of the fact of the existence of such 
“brechoidal” lavas. Mr. Rutley has shown that the celebrated 
porphyry of Djebel Dokhan, the original profido rosso antico, often 
! Wilts Arch. Mag., xxi. 142. 
