152 
THE ROCK ROOM. 
various organisms in boring into, and otherwise destroying, 
rocks. 
On the opposite side of the Case the subject of denudation is 
further illustrated by many specimens showing the weathering of 
rocks — an alteration effected in large measure by the chemical 
action of rain and percolating waters. Thus, it will be seen 
that the felspars of certain granitic rocks are decomposed and 
converted into china-clay ; whilst other specimens show how a 
“ blue-hearted ” rock may become more or less rusty on the 
surface by the production of hydrated oxide of iron, due to the 
effect of the oxygen dissolved in waters which percolate through 
the rocks. 
Finally, an attempt is here made to illustrate certain phases 
of metamorphic action, showing especially the alteration which 
rocks may suffer, when invaded by an igneous intrusion. 
Shales, for instances may be more or less calcined ; sandstones 
may be converted into quartzites ; coal may be coked ; or a com- 
pact limestone may assume a crystalline structure and pass 
into a statuary marble. 
The remainder of the Petrographicai Gallery is occupied by 
a collection of British Rocks, which, for extent and complete- 
ness is unrivalled. In the arrangement of these rocks a 
chronological sequence is observed : the series commencing in 
Wall Case No. 1 with the Archsean, or oldest known rocks, 
from the north-west of Scotland, and tbence proceeding regularly, 
as far as possible in ascending stratigraphical order, through 
the successive geological formations. The Cases along the south 
wall of the Room, numbered 1 to 17 contain specimens illus- 
trating the Pre-Cambrian, the Cambrian, and the Silurian 
rocks. In the upper part of each Wall Case, at too great a 
height for the convenient display of specimens, there is placed 
a geological map of the particular area from which the specimens 
in the Case below have been obtained ; whilst in many of the 
Cases illustrations of the microscopic structure of the rocks are 
introduced, in the form of coloured plates from Mr. Teall’s 
“ British Petrography.” 
Above the range of Wall Cases on each side of the Gallery 
are bold diagrammatic sections, which serve at once to enliven 
the Room, and to illustrate the natural relations of the strata. 
The stratigraphical series is continued in the Wall Cases on 
the opposite or northern side of the Room. To the west of the 
doorway leading into the Gallery is a range of cases (Nos. 18 
to 21 ) devoted to the Devonian rocks and Old Red Sandstone . 
The rocks of Cornwall and Devon, including the “killas,” or 
clay-slate, of the former and the fossiliferous limestones of the 
latter county, are well represented ; and here, as elsewhere in 
the collection, the associated igneous rocks are exhibited, so 
that we find in these Cases representatives of such rocks as the 
granites of Cornwall and Devon and of the volcanic series 
