[ 419 ] 
Avliich is the rxame of the fpot where this caufe^vay is 
lituated, is a large infulated rock, compofed of the fame 
fort of grey granite that is common to the Enganean 
hills, and which I have before defcrihedr^;. The columns 
which form this caufev/ay, partly againft the Hank of 
the rock, and partly round its bafe, are of the fame fub- 
fiance, with the rock itfelf, to which they adhere, as I 
have conftantly obferved in all fimilar groups. They are 
therefore of a compound nature, like the columns of 
Monte Roffo, and differ intirely from the common fort, 
which are moftly homogeneous, or of an unitorm tex« 
ture; as is obfervable in the jointed, as well as iimple fpe- 
cies oibajaltes, I diall take the firft opportunity of fend- 
ing a fragment of one of the newly difcovered columns, 
for the infpeaion of the Society; in the mean time the 
inclofed pieces, which were broken from one of them, 
will ferve to drew, how different their fubftances is trom 
that of the common bafaltic columns. By compaimg 
thefe pieces with the fragments of the columns of Monte 
Roffo which 1 before tranfmitted to the Society, fome 
effential difference will appear between them. Thofe 
of San Biafio, though very hard, are rather porous, of 
a lighter colour than the columns of Monte Roffo, and 
very much refemble a fpecies of lava^ which I have of- 
ten feen. This poroufnefs I alfo remember to have 
once before obferved, and more dgnally too, in fome 
bafaltic columns near Achon, in the province of Au- 
vergne, in France. The pores in the columns of both 
{h) See Aitide II. of this volume. 
thefe 
