ANKERITE VEINS OF LONDONDERRY——LOUIS. 49 
ne HE Ul. 
Insoluble Siliceous Matter... .... 0.57 0.12 0.19 
Mileieh@arbonate.. .. ..., ad ie SO A 49.32 54.96 
Ferrous CPN et HOLE Tegel Bey A DS phe) Daal t 21.92 
PAMOAMOUS oe. ke wt)a- bone iene Oaded, 0.68 1.29 
PEPTIC 55 foe o) a diel Leap Asie esate <o DeA eT DG DY) 21.42 
[i orcs MOS he trace. 1.05 
Q9.75 99.52 100.835 
By taking the mean of these and numerous other analyses, I 
deduce the following for the average composition of Ankerite, 
disregarding the insoluble matter :— 
Caitcre (Car bomate 2)... 5. 53.75 
Ferrous SLi ais ee 22. 2 
EMMOUS (yn Ui pee <hek ei! 70 i. 23 50 
Nancanorns! fe ppipislss achat 0.80 5 
Magnesic “ TLS ROE rae 
100.00 
This would demand for Ankerite the formula 8 Ca Co; + 3 Fe 
Coz + 4 Mg Oo; ; a small proportion of the Iron being replaced 
by Manganese. The composition corresponding to this formula 
fo) I if cap) 
<—— 
Calere Carbonatege :. 0.2. oes 53.90 
Ferrous CE Te IA et eieearncooy Pade ee Nn 23.45 
TATE ScOT SIS He A pee eon ge nr Os 22:65 
100.00 
The correspondence of this composition with that actually 
found is sufficiently close to warrant the above formula for An- 
kerite. It must, however, be borne in mind that all the Carbon- 
ates composing Ankerite are isomorphous, and therefore capable 
of replacing cach other in any proportions ; this circumstance of 
course precludes the possibility of obtaining any universally ap- 
plicable formula for Ankerite. As an example of the irregulari- 
ties produced by isomorphism, J may instance the following 
