Be. - 
- 
Se, 
Cx 
Parr IL.§ ii] ROCK-FORMING MINERALS. 61 
ealeedony may be taken as the type. Minerals in this form have 
probably always resulted as a deposition from aqueous solutions. 
4, Amorphous, having no crystalline structure or form, and 
occurring in indefinite masses, granules, streaks, tufts, stainings, or 
- other irregular modes of occurrence. 
A mineral which has replaced another and has assumed the 
external form of the mineral so replaced, is termed a Pseudo- 
morph. A wineral which encloses another has been called a Peri- 
_ morph ; one enclosed within another, an Endomorph. 
Minerals may either be essential or accessory, original or 
secondary constituents of rocks. A mineral is an essential in- 
gredient when its absence would so alter the character of a rock 
as to make it something fundamentally different. The quartz 
of granite, for example, is an essential constituent of that rock, 
the removal of which would make some other petrographical 
species. All essential minerals are original constituents of a rock, 
but all the original constituents are not essential. In granite, for 
example, topaz, beryl, sphene, and other minerals often occur under 
circumstances which show that they crystallized out of the original 
magma of the rock. But they form so trifling a proportion in the 
total mass, and their absence would so little affect the general 
character of that mass, that they are regarded as mere acces- 
sory though undoubtedly original ingredients. Again, in rocks of 
igneous origin, such as modern lava, the essential ingredients cannot 
be traced back further than the eruption of the mass containing 
them. They are not only original as constituents of the lava, 
but are themselves original and non-derivative minerals, produced 
directly from the crystallization of molten minerals ejected from 
beneath the earth’s crust, though, as Michel Lévy has shown, the 
débris of older minerals may sometimes be traced amidst the later 
crystals of massive rocks.? In rocks of aqueous origin, however, 
there are many, such as conglomerates and sandstones, where the com- 
ponent minerals, though original ingredients of the rocks, are evidently 
of derivative origin. The little quartz granules of a sandstone have 
formed part of the rock ever since it was accumulated, and are its 
essential constituents. Yet each of these once formed part of some 
older rocks, the destruction of which yielded materials for the produc- 
tion of the sandstone. 
The same mineral may occur both as an original and as a 
secondary constituent. Quartz, for example, appears everywhere in 
both conditions; indeed, it may sometimes be found in the twofold 
form even in the same rock, though there is then usually some 
difference between the original and secondary quartz. A quartz- 
felsite, for instance, abounds in original httle kernels, or in double 
1 Some of the “accessory” minerals, however, may be of great importance as 
indicative of the conditions under which the rock was formed. 
'  * Bull. Soc. Géol. France, 3rd ser. iii. 199. See also Fouqué et Michel Lévy, 
“Mincératogic Micrographique,” p. 189. 
