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THE PORTLAND BUILDING STONE. 
By the Rev. J. F. BLAKE, M.A., F.G.S. 
F ROM a builder’s point of view tbe name of Portland Stone 
bas a very different meaning from that it would have in 
the mouth of a geologist. To the former it would signify the 
produce of certain quarries, whence the stone employed in 
building St. Paul’s Cathedral and other edifices was obtained ; 
while to the latter it would indicate the produce in the way of 
limestones more or less suitable to building of a certain period 
of the earth’s history. The first is much the most restricted 
meaning ; for, however great similarity geologists may find in 
the same formation when traced over wide areas, for practical 
purposes, such as the discovery of valuable building stone, it 
is well known, or should be, that the minor changes in a bed 
take place very rapidly : so that it is a rare circumstance to 
find stone, though on exactly the same geological horizon, 
having the same value in quarries a few miles apart. Igno- 
rance of this fact is said to have been the cause of the failure 
in the selection of good samples of dolomite for the construc- 
tion of the Houses of Parliament ; the stone of one quarry was 
recommended, and the substitution of stone from another 
quarry was thought to be immaterial, the stone being geo- 
logically the same. 
In the case of the Portland Stone, though so-called Port- 
land rocks are quarried in many places and used for building, 
in not one of them, except at a quarry near Tisbury, is the 
workable stone on the same geological horizon or of the same 
quality as that which is so famous. By an accurate survey of 
all the localities in which Portland rocks have been worked, 
the details of which have recently been laid before the Geo- 
logical Society, I am enabled to reconstruct, to a certain 
extent, the physical geography of the period, and to assign 
the various beds of workable stone to their proper position 
in the series. 
