150 
THE ROCK ROOM. 
hornblendes, augites, &c. The same arrangement is followed 
Throughout the series ; the student finding in each case the 
mineral, the model, and the microscope section. The essential 
constituents of the common crystalline rocks are followed by a 
series of accessory minerals and secondary products — the whole 
forming a group not large enough to bewilder a beginner, yet 
sufficiently extensive to lay the foundation of sound petrographical 
knowledge. 
From the case of rock-forming minerals the student should 
pass to the range of Wall Cases ( B) erected at the western end of 
the Room, or right hand from the entrance, and containing 
specimens in illustration of the chief Rock types. Here we see 
a rather large selection of rock' specimens, brought together 
without regard to locality, for the purpose of showing the 
principal varieties of rocks. The series commences with the 
igneous rocks , or those which have existed in a more or less 
molten condition. Some of these have consolidated under 
pressure at great depths, and are hence called plutonic, whilst 
others which have solidified rapidly under superficial conditions 
.are termed volcanic. So far as chemical composition is con- 
cerned, we recognise first an acid group, containing rocks rich 
in silica, such as granite, felsite, rhyolite, and obsidian. The 
acid group, in which the silica varies from 75 to 65 per cent, 
is followed by an intermediate group, containing from 65 to 
55 per cent, of silica, and including such rocks as syenite, 
diorite, and andesite, whilst the basic group includes rocks 
poor in silica, like gabbro, dolerite, and basalt, in which the 
silica may be as low as 45 per cent. Finally, an ultra-basic 
group, with less than 45 per cent, of silica, has been formed 
for the reception of the peridotites, picrites, and serpentines. 
Then follows the series of sedimentary rocks, arranged in 
three groups, according as their formation has been due to 
mechanical , organic, or chemical agencies. The metamorphic 
series, though rather ill-defined, is here made to comprehend 
the gneisses, the crystalline schists, and the clay slates. The pre- 
ceding petrological classification, though by no means free from 
imperfection, is .probably the best scheme for the general use of 
the student. 
At the opposite, eastern or left-hand end of the Gallery, is 
a range of Cases (C ; devoted to the exhibition of specimens 
illustrative of Rock structures. In the first compartment we 
find, among others, some beautiful examples of porphyritic 
structure, or that structure in which distinct crystals of certain 
minerals are embedded in a base of finer texture ( see also p. 20). 
Then follows an interesting suite of specimens showing globular 
structures, such as are typically developed in the beautiful 
orbicular diorite from Corsica, known also as Napoleonite. In 
the next Case are varied examples of the structures displayed 
by volcanic rocks, including the vesicular and amygdaloidal 
types ; in the former the lava presents a cavernous texture, due 
to the disengagement of gas or vapour, usually steam, while the 
