48 ANKERITE VEINS OF LONDONDERRY—LOUIS, 
beds of the Silurian formation. They are found of all thick- 
nesses from about the tenth of an inch up to fifty feet; the 
larger deposits are very variable in thickness, much faulted, and 
approximately parallel to each ocher and to the general strike of 
the strata, whilst the rocks between them are frequently 
traversed in every direction by a network of the smaller veins. 
‘These veinlets appear to occur for the most part in the blue slates, 
but the walls of the larger deposits most frequently consist of 
greenish-grey shale. I have not been able to make out any de- 
finite relations between the modes of occurrence of these two 
«lasses of Ankerite veins, | 
Wherever I have been able to examine these deposits, I have 
found them to present very similar characters. Most of my ob- 
servations have been made on the large deposits of Ankerite in 
the right bank of the west branch of Great Village river, which 
has been very extensively quarried: I have, however, no doubt 
but that they will apply equally well to any portion of the 
series of Ankerite veins. 
The following descriptive list includes all the minerals that I 
have up to the present met with in these veins: 
Ist.—Ankerite. This mineral occurs most frequently in the 
massive crystalline state, readily cleavable into rhombohedra, the 
cleavage planes being often very large; sometimes, but more 
rarely, it is eryptocrystalline and granular. I have, however, 
also found it in true crystals, lining the walls of a small fissure 
in the vein. The crystalline form is the simple primary rhom- 
bohedron, very minute, the largest erystal not being over $ inch 
in length, and the faces too dull for measurement. The colour of 
Ankerite before it has been exposed to the atmosphere is pure 
white, but, owing to the rapidity with which its protoxide of 
iron is per-oxidized, it is usually found of a yellow or brownish 
colour, 
Its specific gravity is 2.998. , 
The following are analyses of three specimens of the pure 
mineral; Nos. I and IL were white, and No. III of a brownish 
tinge :— 
