PORPHYRY, &C. 
21 
character, and hence embracing many kinds which in chemical 
composition are widely distinct. It is convenient, however, to 
restrict the term porphyry to acid porphyritic rocks, containing 
orthoclase-felspar, and to apply the term porphyrite to basic ' 
rocks, of similar texture, with plagioclase-felspar. The ancient 
red porphyry of Jebel Duchan in Egypt, which, from its purple 
colour, originally gave its name to this group of rocks, is a 
hornblende-porphyrite. The ancient green porphyry of Greece, 
with large pale green crystals of plagioclase, is technically a 
diabase-porphyrite ; and the green porphyritic rock of Lam bay 
Island, near Dublin, represented by the cube No. 77, is of 
similar character. 
Many of the Cornish and Devon granites in the collection are 
eminently porphyritic, and we may especially notice the 
pedestals and columns Nos. 61, 75, and 176. 
The sarcophagus for the Duke of Wellington, in St. Paul’s, 
is formed of one huge mass, of the same character as the 
red and black variety in the column No. 170, and in the bases 
of the pedestals Nos. 75 and 81. This beautiful rock, which, 
from its locality, has been called by Prof. Bonney LuxulUcinite , 
consists of large crystals of pink orthoclase-felspar, in a dark 
ground-mass of schorl, or black tourmaline, with a small 
proportion of quartz. 
Upon reference to a geological map of Cornwall a number of 
bands will be seen traversing both the granite and slate rocks, 
and having a main general direction from the north of east to 
to the south of west. These represent dykes of what is locally 
termed Elvan. The miners include under this name a great 
variety of rocks, some of which resemble fine-grained granites, 
with little or no mica {microgranite). Frequently the el vans 
become porphyritic, being chiefly composed of a fine felspathic 
or quartzo-felspathic base, containing crystals of felspar and 
quartz, sometimes with schorl and occasionally, though rarely, 
mica. Such rocks pass under the names of quartz-porphyry 
and quartz- folsite , in allusion to the presence of porphyritic 
crystals of quaitz, often bi-pyramid al and sometimes so rounded 
as to appear as mere blebs ; while if felspar only be present as the 
porphyritic constituent, the elvan becomes a felspar-porphyry. 
Sir Henry De la Beche remarks of the Cornish elvan s : “For 
durable stone the harder elvans of this district, particularly 
when of good cream and other light colours, may be considered 
as the best building materials in it : their durability and appear- 
ance may be seen in many churches and old mansions, where 
the finer carvings of the ornamental parts are as sharp as the 
day they were put up.” 
In the Table Case III. and in the Wall Cases, Nos. VII. and 
VIII., will be found specimens of elvans from Mayen, Land’s 
End ; Marazion, near Penzance ; Breague, near Helston ; Por- 
kellis, Wendron; Roscrow and Trevailes, Penryn; Newhaven, 
Truro ; near Newquay ; Pentuan, and Dowgas Mine, St. Austell ; 
