70  GEOGNOSY. [Boor TL *3 
in ordinary rock-forming minerals, but in such minute proportions as 
_to have been generally neglected in analyses. Their presence in the — 
rocks of a district is sometimes shown by deposits of the hydrous 
Oxide in the forms of psilomelane and wad. ‘These deposits some- 
times ‘take place as black or dark brown branching, plant-like or 
dendritic impressions between the divisional planes of close-grained 
rocks (limestone, felsite, &c.) sometimes as accumulations of a black 
or brown earthy substance in hollows of rocks, and occasionally as 
deposits in marshy places, like those of bog iron ore. ! 
SiiicatEs.—Lhese embrace by far the largest and most im- | 
portant series of rock-forming minerals. Their chief groups 
are the anhydrous aluminous and magnesian silicates embracing 
the Felspars, Hornblendes, Augites, Olivines, Micas, &e., and the 
hydrous silicates which include the Zeolites, Clays, tale, chlorite, 
serpentine, &c. . 
The family of the Felspars forms one of the most important of all - 
the constituents of rocks, seeing that its members constitute by much 
the largest portion of the plutonic and volcanic rocks; are abun- 
dantly present among many crystalline schists, and by their decay 
have supplied a great part of the clay out of which argillaceous sedi- 
mentary formations have been constructed. | 
The felspars are usually divided into two series. 1st, The ortho- 
clastic or monoclinic felspars, consisting of two species or varieties, 
Orthoclase and Sanidine; and 2nd, The plagioclastic or triclinic 
felspars, among which, as constituents of rocks, may be mentioned 
the species albite, anorthite, oligoclase, andesine, labradorite, and 
microcline. 
Orthoclase, monoclinic, commonly in twins of the Carlsbad 
form, the suture of which can often be seen with the naked eye on 
the abundant and perfect cleavage planes; occurs in well developed 
erystals in many porphyritic rocks, also in the drusy cavities of 
granites; but more frequently, as a constituent of rocks, pre- 
sents incomplete crystals, and even more or less rounded or irregular 
crystalline forms. Colourless, but more usually white, grey, or 
pink, the sanidine being clear and glassy, the orthoclase somewhat 
turbid. Normal composition, Silica 64:6, alumina 185, potash 16°9, 
but with small and variable proportions of lime, iron, magnesia and 
soda ; scarcely affected by acids. Under the microscope recognizable 
from quartz by its characteristic cleavage, twinning, turbidity, and — 
frequent alteration.’ A peculiar lattice-like network of interlacing | 
lines, or a fine parallel striping, may be observed on a fresh 
cleavage face of some varieties of orthoclase, such as that of the 
well known red granite of Upper Egypt. This must not be con- 
founded with the characteristic lamellation of the triclinic felspars. 
It appears to arise In many cases, if not always, from the crystalliza- 
tion together of parallel or intersecting lamine of some other felspar 
(albite for example) with the orthoclase. 
* On microscopic determination of felspars, see Fouqué et Michel-Lévy op, cit. pp. 209, 227. 



