24 i\li'. H. C. Sorby on Slaty Cleavage, 



therefore group the constituents accordingly, and with reference 

 to the principles just described. 



A yellowish, even-grained crystalline limestone : — 



Stonehouse No. 2. (Chemical analysis.) 



Carbonate of lime 54*3^ ^^,^^^ 



Carbonate of magnesia . . . . . 447 ^^^^^^^^.^^ 

 Carbonateofprotoxideofii-on (oxidized) -8 J 



Peroxidized pyrites "2 



In this, then, the composition is that of a true dolomite, with 

 equal equivalents of the two carbonates and no excess of car- 

 bonate of lime. 



A yellow, more marly-looking specimen than the above : — 



Stonehouse No. 5. (Chemical analysis.) 



Carbonate of lime 48*5^ ack-o <• . 



Carbonate of magnesia 39'8 



.q r dolomite. 



Carbonate of protoxide of iron (oxidized) 

 Excess of carbonate of lime .... 10"8 



100-0 



Here then there is 10"8 per cent, of carbonate of lime in excess. 

 The cause of this is often well seen in sections ; for they show 

 that the rock has been originally an organic deposit, and that 

 crystals of dolomite have been formed in some parts of it, and 

 the rest has remained in its original condition. As an illustra- 

 tion of this, I select one from Paignton, whose physical consti- 

 tution is very similar to Plymouth No. 3 given above, with the 

 addition of ciystals of dolomite and brown-spar. 



Paignton No. 1. (Chemical analysis.) 



Inorganic clay 1-2 



Carbonate of lime 4'01 



Carbonate of magnesia 2'9 [ 7*3 true 



Carbonate of protoxide of iron (oxidized) . "2 [dolomite. 



Carbonateof protoxideofmanganese (oxidized) '2 J 

 Excess of carbonate of lime . . . ... 91'5 



100-0 



I now give one which contains a good deal of inorganic clay, 

 and is much reddened with peroxide of iron, seen by the micro- 

 scope to exist as very minute grains in the solid crystals of do- 

 lomite, and also as larger separate crystals, probably of peroxidized 

 pyrites. 



