100 The Microscopical Examination [| Menenis Microsconrat 
crystals of felspar in a compact felspathic base. There is in fact 
a gradual passage from the compact Felstones to the Porphyrites ; 
so that it appears probable that the amorphous base of all such 
rocks is simply the silicious magma or paste from which distinct 
minerals would have separated had the circumstances under which 
they were formed been favourable for crystallization to have taken 
place. As a typical example of a widely-distributed class of rocks, 
I will take for description the well-known basaltic rock of the 
Rowley Hills. An examination of thin sections shows that it con- 
tains a triclinic felspar, augite, magnetic oxide of iron, a little olivine, 
and a few crystals of apatite. The felspar is known to belong to 
the triclinic system, as 1t exhibits the characteristic strize when ex- 
amined by polarized light. Augite occurs in minute black shining 
crystals, which appear bright brown, or occasionally green when 
in thin sections; it cannot be mistaken for any other mineral 
except hornblende, from which it is distinguished by a marked dif- 
ference in the angles; it has also a clear glassy appearance, while 
hornblende is either distinctly fibrous in texture, or exhibits lines 
or cracks running parallel with the principal cleavage plane: very 
frequently hornblende is green, while augite is yellowish brown, 
but this does not always hold good. The magnetite occurs in 
minute grains, or as thin lamin, both being black and opaque. 
The apatite is seen in long acicular hexagonal crystals. The olivine 
occurs in crystals, and also in irregular grains, of a clear yellowish 
green colour ; it is, however, rare in this state, being nearly always 
altered to a dull dark-green mineral. In addition to the above 
minerals, which must be regarded as the original constituents of 
the rock, there are also one or two zeolites, calcite, and a chloritic 
mineral, all of which are found filling cavities, and are secondary 
formations. 
The use of the microscope is by no means confined to the dis- 
crimination of minerals; with its assistance we may learn many 
facts, as to the mode of formation of rocks, the order in which the 
various minerals crystallized, and the alterations which have been 
frequently caused by the removal of mineral matter, and its replace- 
ment by another of different chemical composition. In the Rowley 
rock, the minute crystals of apatite penetrate both the felspar and 
augite ; the latter also encloses crystals of felspar and magnetite: 
the augite crystallized therefore after the others had been formed. 
The olivine contains grains of magnetite only, and was probably the 
second to crystallize. 
Cases are not uncommon in which crystals have caught up 
portions of the surrounding mass while in the act of formation, and 
other facts indicate very clearly the actual condition of the mass at 
the moment of crystallization. For example, in a section of Pitch- 
stone from Planitz, containing crystals of felspar, the minute opaque 
