PKOGRESS OF MICROSCOPICAL SCIENCE. 
41 
or less crystalline or amorphous magma by which they are surrounded, 
and which had its origin at the time of the consolidation of the erup- 
tive rock. This distinction is well marked in porphyritic rocks. 
These rocks are generally composed of well-developed crystals im- 
bedded in a more or less crystalline paste. This paste is the magma 
of consolidation, while the crystals are ancient. In ancient granites 
the crystalline elements of the magma of consolidation have dimen- 
sions comparable with those of the ancient crystals, so that it is diffi- 
cult to distinguish them with the naked eye. The ancient crystals 
are black mica, amphibole, oligoclase, orthose and quartz, and the 
magma orthose and quartz. Eecent quartz is moulded on earlier 
crystals ; ancient quartz found in a mass that was still fluid exhibits 
bipyramidal grains. This form, which some geologists have con- 
sidered characteristic of porphyries, merely indicates the presence of 
ancient crystals. The ancient crystals in granites wijih white mica, 
and elyans, are chiefly formed of black mica (not abundant), quartz, 
oligoclase, and orthose ; the magma being orthose, quartz, and white 
mica. The white mica is the latest crystallized, from which it results 
that the recent quartz was often able to crystallize in its proper form, 
and thus, like the ancient quartz, exhibits bipyramidal grains. On 
the borders of massive granites with white mica, or when the rock is 
injected with thin veins, the magma is finer, and the texture por- 
phyritic. This constitutes elvans, which appear, under the microscope 
completely crystallized and formed of very small elements of quartz 
and white mica. In granulites the ancient crystals are rare, and the 
magma composed of united grains of felspar and quartz. In spots of 
certain dimensions' the crystalline elements of felspar arrange them- 
selves parallel to each other as if to form a more developed crystal. 
The ancient crystals of the porphyritic group do not serve to classify 
them. The magma, however, is sometimes entirely crystalline, as in 
granulites, while in the Permian porphyries it is more or less amor- 
phous, and in optical properties approaches the vitreous rocks. These 
groups M. Levy distinguishes as granulitic and petrosilicious por- 
phyries. In granulitic porphyries he finds the ancient crystals com- 
posed of black mica, amphibole, pinite, quartz, oligoclase, and orthose. 
The magma closely resembles that of the granulites, but the elements 
are generally smaller. Frequently round the ancient crystals are 
mixtures of orthose and quartz, reproducing, on a small scale, graphic 
pegmatite. While it is difficult to establish sharp distinctions 
between difierent granulitic porphyries, the more recent are usually 
characterized by a finer microgranulite, and by a micropegmatite of 
smaller components. In the last rocks of the series, the micro- 
pegmatites only form aureoles, or fibrous radiating globules, difficult 
to resolve under the microscope ; but the character of the orientation 
of the recent quartz is always dominant, and the aureoles and globules 
become extinct when the Nicol prisms are crossed. In petrosilicious 
porphyries (eurites of Griiner, Permian porphyries) the magma ex- 
hibits a more or less considerable proportion of amorphous paste, 
extinguished in all directions by the crossed prisms. They also pre- 
sent the texture called "fluid" — that is to say, the debris and the 
