te vr . - * 

62 GEOGNOSY. 
pyramids of the mineral often enclosing fluid cavities, while the 
secondary or accidental forms occur in veins, reticulations, or. 
other irregular aggregates, distinguished by a peculiar chequered 
structure in polarized light, and by an absence of the crowded — 
cavities so characteristic in the quartz of igneous rocks, 
Accessory minerals frequently occur in cavities where they have — 
had room to erystallize out from the general mass. The “drusy ” 
cavities or open spaces lined with well developed crystals found in 
some granites are good examples, for it is there that the non-essential 
minerals are chiefly to be recognized. The veins of segregation 
found in many crystalline rocks, particularly in those of the granitic 
series, are further illustrations of the original separation of mineral 
ingredients from the general magma of a rock (see p. 132). In some 
cases minerals assume a concretionary shape, which may be ob- 
served chiefly though not entirely in rocks formed in water. Some 
minerals are particularly prone to occur in concretions. Siderite or 
ferrous carbonate is to be found in abundant nodules mixed with. 
clay and organic matter among consolidated muddy deposits. Cal- 
cite or calcium carbonate is likewise abundantly concretionary. 
Silica in the forms of chert and flint appears in irregular concretions, 
in old calcareous formations, composed mainly of the remains of 
marine organisms, 
Secondary minerals haye been developed as the result of sub- 
sequent changes in rocks, and are almost invariably due to the 
chemical action of percolating water, either from above or from 
below. Occurring under circumstances in which such water could 
act with effect, they are found in cracks, joints, fissures, and other 
divisional planes and cavities of rocks, These subterranean channels, 
frequently several feet or even yards wide, have been gradually 
filled up by the deposit of mineral matter on their sides (see the 
Section on Mineral Veins). The cavities formed by expanding ~ 
steam in ancient lavas (amygdaloids) have offered abundant op- 
portunities for deposits of this kind, They have accordingly been © 
in large measure occupied by secondary minerals (amygdules), such 
as calcite, caleedony, quartz and zeolites. 
In the succeeding description of the more important rock- 
forming minerals, attention will be drawn to physical characters, 
such as crystalline form, hardness’ (H.), and specific gravity (Gr.) ; 
chemical are pine i modes of occurrence, whether original or 
secondary ; and modes of origin, whether igneous, aqueous, or organic ; 
waged tig that is, the various minerals which any given mineral 
1a8 replaced, while retaining their external forms, and likewise those 
which are found to have supplanted the mineral in question while in 
the same way retaining its form—a valuable clue to the internal: 
’ The seale of hardness in use o miner ists is divi i La 4 
denoted by the name of somo minoral: 1 Tale. 2, Weck-ealt 8 Gnas ee 
5. Apatite, 6. Orthoclase. 7. Quartz. 8. Topaz. 9. Corundum. 10. Diamond. 
A mineral which is scratched with the same ease as quartz is said to have H. 7: a 
. : . . . 7 d= 
mineral which scratches fluor-spar, but is scratched by apatite, is between H.4 and H.5. 

