
Parr IX.§1.] | MINERAL VEINS. 593 
~ (1.) Massive.—showing no definite arrangement of the contents. 
This structure is especially characteristic of veins consisting of a 
single mineral, as of calcite, quartz, or barytes. Some metalliferous 
ores (pyrites, limonite) likewise assume if. 
(2.) Banded, or in parallel (and usually duplicated) layers. 
In this common arrangement, each cheek (a a, Fig. 303) may be 
coated with a layer of the same material (6 6), followed on the inside 











| | | aa . he 
| lk SIU aa * : -— 
ANAS @ $272 — 
it = S= — 
Msi 
1 b Cc 
Fid. 303.—Srction or MINERAL VEIN WITH SYMMETRICAL DISPOSITION OF 
DUPLICATE LAYERS. 



by another layer, ¢c, and so on to the centre, where the two opposite 
walls are finally united by the last zone of deposit (¢). Hven where 
each half of the vein is not strictly a duplicate of the other, the 
same parallelism of distinct layers may be traced. 
(3.) Brecciated, containing angular fragments of the sur- 
rounding rock (or “ country,”) cemented in a matrix of veinstones or 
ores. It may often be observed that these fragments are completely 
enclosed within the matrix of the vein, which must have been 
partially open and the matrix still in course of deposit when they 
were detached from the parent rock. 
(4.) Drusy, containing or made up of cavities lined with erystal- 
line minerals. The central parts of veins frequently present this 
structure, particularly where the minerals have been deposited from 
each side towards the middle. 
(5.) Filamentous, having the minerals disposed in thread- 
like veins; this is one of the commonest structures. 
Metallic ores occur under a variety of forms in mineral veins. 
Sometimes they are disseminated in minute grains or fine threads 
_ (gold, pyrites), or gathered into irregular strings, branches, bunches, or 
leaf-like expansions (native copper), or disposed in layers alternating 
with the veinstones parallel with the walls of the vein (most metallic 
ores), or forming the whole of the vein (pyrites, and occasionally 
galena), or lining drusy cavities, both on a small scale and in large 
chambers (hematite, galena). Some ores are frequently found in 
association (galena and blende), or are noted for containing minute 
proportions of anotlier metal (argentiferous galena, auriferous 
pyzites). 
Successive in-filling of veins.—The symmetrical disposition 
2Q 
