METAMORPHIC.
The second class of rocks are the metamorphic, exclusive of the volcanic, which are confined to no period, but are distributed through all ages and formations and are the production of all periods, even to the present. The metamorphic, gneissic, or stratified crystalline rocks repose on the granites, or plutonic rocks, which have no stratification, but are massive and devoid of all regular cleavage. But the gneiss, though approaching the granites in appearance and constituents, is irregularly stratified, and evidently a sedimentary rock, or aqueous deposit, but metamorphosed or crystallized by the action of heat; or, in other words, they were formed in boiling water, and are the results of volcanic agencies and the debris of subaqueous plutonic formations. The metamorphic rocks, however, are not all gneiss: they are widely distributed, and exist in various shapes and under various names, such as hornblende, mica, talcose and clay slates, talcose and harnblendic gneiss, quartz, crystalline limestone, mica schist, chloritic schist, &c. &c. These rocks exist exclusively in North America
on the Atlantic slope, on the Laurentian water-shed, north of the great lakes, in the Rocky Mountains, and in the great mountain chain of Cali fornia and Oregon. They contain most of our veins of gold, copper, and magnetic iron ores, and particularly the copper of Lake Superior and Tennessee and the gold-bearing veins of the Southern States and California.
In Pennsylvania, these formations have been divided by Rogers into the "ancient metamorphic" and the "semi-metamorphic," representing the hypozoic and azoic rocks, or those destitute of the ancient life or fossiliferous remains, and those (hypo) underlying, or beneath. The semi-metamorphic, or azoic, are at the base of the palceozoic, or those rocks which entomb the ancient life, and which are replete with fossiliferous remains.