22 
THE HALL. 
Withiel, Lanivet, St. Neots, and Tremore, near Bodmin ; St. 
Dennis ; Camelford ; and Meldon, Okehampton, Devon. 
The term “ Greenstone ” has been rather loosely applied to 
many igneous rocks, which have acquired a dark green tint as 
a result of the alteration of some of the mineral constituents, 
and the consequent formation of chloritic products. Some of 
these greenstones should be technically termed diabase and 
others diorite. A diabase is an altered basic rock, which origi- 
nally contained plagioclase and augite ; whilst a diorite is a less 
basic rock, containing plagioclase and hornblende, or some other 
ferro- magnesian silicate, often associated with free quartz. The 
polished bust of Bubastis (No. 132), placed near the western 
entrance to the theatre, was skilfully carved, after an Egyptian 
statue in the British Museum, by the late Mr. C. H. Smith, in 
a diabase from Llanwnda, near Fishguard, in Pembrokeshire. 
The well-known Pen-maen-mawr stone (No. 151, beneath Case 
VII.), which occurs as an intrusive mass near Conway, in 
Carnarvonshire, and from its toughness forms a valuable paving 
material, is petrographically an enstatite-diorite ; enstatite being 
the name given to a rhombic pyroxene. 
The rocks called syenite contain orthoclase-felspar associated 
with hornblende, or rarely with some other ferro-magnesian 
mineral, such as augite or biotite; and hence syenite differs 
from diorite mainly in the character of its felspathic constituent. 
The word is derived from Syene, the ancient name of Assouan 
in Upper Egypt, where a granite containing hornblende and 
biotite was quarried ; and it was formerly the practice to refer 
to hornblende-granites as syenitic rocks. True syenite is by no 
means a common rock in Britain, and is not used as an 
ornamental stone. 
Marble. 
It is a common practice to comprehend under the name of 
marble most stones which are capable of receiving a polish and 
of being applied to purposes of decoration, the term being thus 
made to include such substances as serpentines, porphyries, 
alabasters, and other ornamental stones. It is convenient, how- 
ever/to limit the term to those varieties of limestone which are 
sufficiently hard and compact to be susceptible of polish and of 
application to ornamental uses. A typical marble is an aggregate 
of granular crystals of calcite. 
Although by no means restricted to any particular strata, yet 
the marbles of this country are usually obtained from the 
palaeozoic rocks, being especially abundant in the Carboniferous 
and Devonian systems. The Carboniferous or Mountain Lime- 
stone— which runs round the margin of many of our coal-fields, 
and rises in our northern counties as a broad ridge or anticlinal 
curve forming the Pennine chain — furnishes valuable marbles in 
certain districts, especially in Derbyshire and on the borders of 
