ANALYSES OF MAGNESIAN LIMESTONE. 123 
constantly percolates through these beds of limestone undoubtedly 
has much to do with their physical and chemical change, and with 
the object of ascertaining how these changes were brought about, 
the following analyses were made :— 
No. 1 No. 2 No, 3 
Silleakessccesecees W623 ieaneiseees LOOOwieseccsesceests 4:50 
Alumina ........ O;6 Ose tescacsessets DG Ossecdeceeesteus 0:70 
Reroxid elironeesi025 Osceceacceseee ce Ae esswesveccesiee 0:80 
Carb. Magnesia 4°79.......0.000005 BBs onoooosongss00 27:00 
Pree IIN vee Olga Osmenctescacactdee OY O asodoaabeanda 67:00 
99°01 100-64 100:00 
It will be seen that No. 2 (which is the soft clay-like limestone) 
contains the most silica and the most magnesia, but it is to the 
silica which I wish to draw your attention chiefly. 
Limestones containing silicate of lime are easily acted on by 
carbonic acid contained in the air or in the water which percolates 
through them. The unaltered lower bed (No. 3) contains some 
quantity of silica, and I think it extremely probable that the 
soft clay-like bed (No. 2) has at one time been exactly similar 
in chemical composition to this lower bed, and that the water 
with which it is so largely charged, and which contains carbonic 
acid, may have decomposed the silicate of lime, removing the 
lime in solution, and leaving the silica behind, and in proportion 
as the lime is gradually removed the percentage of magnesia 
and silica will increase, which appears from the analysis to be the 
case. The cause of change, therefore, in this so-called middle 
bed, I think, may be accounted for, by the removal of the lime 
existing as silicate of lime, and in this way producing a soft and 
porous mass such as we find. 
The action of water upon all rocks is well shown to be very 
great, but upon some limestone rocks it is something remarkable 
Itis estimated, for instance, that the Rhine running through lime- 
stone rocks by its action on the silicate of lime in these rocks and 
otherwise, carries annually into the sea some 370 tons of limestone. 
A question like this soon makes itself of practical importance. 
Professor Ansted has recently told us in one of his lectures that 
the quarries of Dolomite, of which the Houses of Parliament are 
built, produce stone, apparently from the same workings, which 
is very differently acted on by the weather; and at Marsden we 
