CHEMICAL EXAMINATION OF ROCKS AND MINERALS. G9 



The composition in 100 parts of the Howth dolomite may be thus repre- 

 sented as follows : — 



Carbonate of lime 53*897 



Carbonate of magnesia 43*610 



Protoxide of iron 1*403 



Residue / 0chre y ^ °" 735 



itesmue <^ Fibrous CJuartz . 196 



Peroxide of manganese (in variable quantity). 



99*841 

 3. Maynesian Conglomerate from Downhill, Go. Londonderry. — This rock, 

 which has been improperly called hydrocarbonate of magnesia, is formed of 

 spherical nodules encircled by a greenish paste, composed generally of car- 

 bonate of iron and of a partially decomposed ferro-magnesian silicate. 



The composition of the part of this conglomerate soluble in dilute hydro- 

 chloric acid varies very much, as does that of the residue likewise. It some- 

 times acquires the character of true dolomite ; and, according to Prof. Oldham, 

 it contains crystals of bitter-spar disseminated through the mass. The fol- 

 lowing Table, containing the results of two analyses of the soluble part, will 

 show the character of the variation alluded to: — 



I. II. 



Carbonate of lime .... 63*700 70-863 



Carbonate of magnesia . 21*325 17*481 



Protoxide of iron .... 3*400 Mil 



Residue 8*875 0*896 



Water 2*700 (by difference) 9*600 (experimentally). 



100*000 99*951 



The following results, obtained by Dr. Apjohn, bear out what has been said 

 above, that the proportion of magnesia to lime sometimes reaches that ob- 

 served in true dolomite : — 



Carbonate of lime 54*8S 



Carbonate of magnesia 38*23 



Carbonate of iron 5*93 



Silex and loss 0*96 



100-00 

 The residue left after the dissolution of the carbonates in dilute hydro- 

 chloric acid, is generally of a variable greenish colour and a spongy texture; 

 it contains a quantity of water, and also some organic matters. 



The following Table gives the results of analyses of three specimens of this 

 residue : — 



I. II. III. 



Silica 40*371 60*725 70*532 



Magnesia , 13*719 5*656 4*259 



Lime 0*251 



Protoxide of iron 6*913 5*461 5*218 



Alumina 0*216 2*557 0*473 



Water and organic matter, "1 3g . 781 25 . 350 19 . 5J8 



&c. by difference J 



100*000 100*000 100*000 



In examining one of those residues with a lens, I found a perfect octahe- 

 dron of red oxide of copper. 



