PORPHYRY. 229 
This rock and its various uses, have been already 
described (132). It generally occurs in shapeless 
masses and beds ; and seldom in distinct strata. It is 
found in Cornwall, the island of Anglesea, and several 
parts of Scotland; but it rarely forms mountains. 
Ores of lead, silver, and copper, are sometimes found 
in serpentine. 
261. PORPHYRY is a primitive rock, consisting of 
quartz (76) or felspar (1 10), or both, embedded in a solid and 
compact cement or ground. 
The ground or basis of porphyry varies in the different kinds. 
In some it is claystone, in others pit chslone, hornstone, or com- 
pact felspar. 
When not covered by other formations, porphyry 
sometimes forms single rocks ; but it never constitutes 
elevated mountains. It occurs in beds of considerable 
magnitude, but never appears in distinct and well-de- 
fined strata. 
There are many beautiful and splendid works in 
porphyry. Obelisks, statues, and columns, wrought in 
it, have had great celebrity. It is susceptible of a 
polish as high as that of marble, but is so hard, that the 
expense of working it has caused it to be much neglected 
by the moderns. This hardness, however, renders it 
very durable, and also constitutes it a material of great 
utility for mortars, slabs for grinding colours upon, and 
for several other purposes. 
Porphyry was much esteemed by the ancient Egyp- 
tians ; and Pliny informs us that the procurator-general 
in Egypt, under Claudius Csesar, brought thence, for 
that Emperor, certain statues of porphyry, which he 
conceived to be very valuable : this act, however, was 
not much approved, and the example was not followed 
by any other Roman. 
The principal quarries of porphyry are in Egypt ; 
but this stone is also found in Italy, Germany, and 
various parts of the European continent. It may be 
traced from Norway to the borders of the Black Sea, 
