16 
THE HALL. 
Tlie screen (B.) between pilasters IS os. 30 and 42 exhibits a 
large central panel of marble from the Mumbles, Swansea, the 
smaller panels being of Ipplepen marble, Devon ; while the 
cornice is of Lizard serpentine, and the remainder of Derbyshire 
marbles. 
In screen (C.) occupying the space between pilasters Nos. 42 
and 46 the circular centre is of Derbyshire rosewood marble, 
and the remaining panels of crinoid, coral, and other varieties 
of Derbyshire marble, whilst the ground in which the whole is 
inlaid is of Fauld alabaster, and the base of reddish hornblende- 
granite from Mount Sorrel, Leicestershire. 
Scotch granites and marbles occupy the screen (D.) in the wall- 
space between pilasters Nos. 46 and 50. On the base of grey 
granite from Kemnay (Aberdeenshire) rests a moulding of red 
Corrennie granite, above which is a dado of grey granite from 
Cairngall, near Peterhead, surmounted by a cornice of pink 
granite from the Isle of Mull. In the upper part the ground is 
of red granite from the Stirling Hill quarries, near Peterhead, 
whilst the central circular panel is of Strathdon marble (Aber- 
deenshire), and on each side of this is a panel of hornblendic 
porphyrite from May on, near Huntly, Aberdeenshire. The 
remaining panels, which in most cases correspond on opposite 
sides, are, commencing from below, of Kingswell porphyry 
(Aberdeenshire) ; Glen Tilt marble (Perthshire) ; Sutherland shire 
marble on the left hand, and Portsoy serpentine on the right ; 
Tiree marble, Hebrides ; and Rubislaw granite, Aberdeenshire. 
The granites are described below. For a description of 
marbles, see p. 22-; of the alabaster, p. 28 ; and of the serpentine, 
also p. 28. 
While examining the contents of the Hall it should be borne 
in mind that the main object of this section of the museum is 
almost purely technological, and hence the geological student 
who wishes to study the physical characters of our rocks rather 
than their industrial uses must be referred to the series of British 
rock specimens in the Petrographical Room on the Upper Floor 
(p. 149). 
Granite. 
This rock, in its typical varieties, is a crystalline aggregate 
of the three minerals, felspar (p. 115), quartz (p. Ill), and mica 
(p. 117). The felspar, which usually forms the chief constituent, 
is in most cases the common potash-felspar called orthoclase ; 
but this is frequently associated with other species, such as 
microcline, albite, and oligoclctse. The orthoclase often occurs 
in large well-formed crystals embedded in a fine-grained base, 
thus producing the beautiful porpkyritic granites, or granite- 
porphyries , of which some fine examples are exhibited (p. 21). 
Sometimes two kinds of mica are present, one being a pale silvery 
mineral called muscovite, while the other is the dark brownish* 
black mica termed biotite (p. 117). A practice has lately grown 
up, following German precedents, of calling a biotite-granite, or 
