Brar.— On the Building Materials of Otago. 111 
occasional beds of silt and sand, the stone cannot fail to contain impurities. 
Even after the deposit has taken place the stone may be altered by mechani- 
cal and chemical agencies, there being a peculiar relationship between the 
lime and clay in hydraulic limestone that seems to be easily affected by 
external causes. A good illustration of the influence of its surroundings 
on the character of the stone is found at Mr. Macdonald’s quarries, Otago 
Peninsula. The rock is much shattered, and divided into large blocks by 
“backs ” running through it in all directions. The blocks in one of the 
beds produce two distinct varieties of stone, the analyses of which are given 
in. Nos. 18 and 15, Table III. The light-coloured stone oceupes from two to 
three feet of the outside of the block, and gradually merges into the dark 
one which composes the heart ;—they vary little in consistency, but, as will 
be seen from the table, there is a great difference in their composition. 
Assuming the block was originally homogeneous, of which there can be 
little doubt, we find that the crust has lost 4 per cent. of carbonate of lime 
-and 24 per cent. of carbonate of magnesia, while, on the other hand, it has 
gained 2 per cent. of iron in addition to the increased percentage of silica 
and alumina due to the abstraction of the lime and magnesia. 
Hydraulic limestones are generally compact in texture and dark in 
colour, grey, blue, drab, or brown being the prevailing colours ; white indi- 
cates pure lime. It does not, however, follow that all the dark-coloured 
limestones are hydraulic, for they may contain sand and other insoluble 
matters that neutralize the effect of the clay; and the darkest of all lime- 
stones—black marble—is almost pure carbonate of lime. Still, a rule may 
be established in a negative manner by saying that no white limestones 
produce hydraulic lime. 
Notwithstanding the advance made in all the practical sciences within 
the last few years, there is still a doubt as to the causes that produce the 
setting and hardening of lime and cement mortar, The old theory was 
that all mortars hardened by the absorption of carbonic acid from the 
atmosphere, and it was supposed that in time the quantity absorbed would 
equal that expelled in burning, so that the mortar would revert to its 
original carbonate, and become again a limestone. It is true that rich 
limes will not set without carbonic acid. The mortar in the inside of a 
bastion at Strasbourg was found, after 160 years, to be quite soft, and the 
same thing was observed in a masonry pillar, nine feet in diameter, at St. 
Peter’s, Berlin, its age being 80 years. It has also been found by experi- 
ment that a mortar of rich lime will not set in the exhausted receiver of an 
air pump. But carbonic acid alone will not perfect the hardening process, 
consequently it is supposed to be assisted by the crystallization of the 
carbonate of lime between and around the particles of the aggregates. 
