a 
50 ANKERITE VEINS OF LONDONDERRY—LOUIS. 
analysis of a specimen of white cryptocrystalline Ankerite, 
which contains much more Lime, and also less Magnesia in pro- 
portion to the Iron than the normal mineral :— 
Insoluble: Matter 0% 22322. 0.53 
Caleie Carbonatetc, wc oo eclos (E23 
Ferrous EF a AN arte trea att 16.41 
Mancanous ys sys cele euerer 2.65 
Maomesio sO) eee neers oe 9.34 
100.16 
The formula for this structure is 18 Ca Cos + 3 Fe Coz + 2 
Mg Cos, It appeared to be perfectly homogeneous in structure, 
and may possibly be Ankerite altered by the action of water 
carrying Calcic Carbonate in solution. 
2nd,—Sideroplesite—(Classed by Dana as a variety of Si- 
derite)—This mineral occurs in the Ankerite quarry in small 
veinlets penetrating the mass of the Ankerite, but appears to be- 
come more abundant in the deeper lying parts of the deposits ; 
thus, in the upper levels of the West Mines, Sideroplesite and 
Ankerite are found in irregularly interlacing veins and masses, 
in about equal proportions, as will be shown by an analysis to 
be given below, while in the lower levels of the same mine very 
large deposits occur, containing only here and there small vein- 
lets and patches of Ankerite; so extensive indeed are these 
deposits, that if they hold in depth, as they now promise to, they 
will become of high economic importance. I have never seen 
any crystals of Sideroplesite, but it is always highly crystalline, 
although the cleavage planes are smaKer than in Ankerite, and, 
instead of being continuous through large masses, are inclined in 
all directions, so that a fractured surface shows a number of 
small and irregularly divergent cleavages. Its colour is pearl 
grey, but on exposure to the air it oxidises with great rapidity, 
assuming a brownish tinge. 
Its specifie gravity is 3.523. 
The following are analyses of some characteristic speci- 
mens :— 
