The thousands or tens of thousands of feet of rock which once overlay the schists and still more the granites and gneisses must have been slowly removed by erosion, for there was no other way to get rid of them.
In fact, the history of the structures of the charnockite series is the history of the most primitive gneisses in all parts of the world, for which we cannot pretend to have as yet any thoroughly satisfactory explanations to offer.
In India the system is represented by the Bundelkhand gneiss and the central older gneisses of the Himalayas.
In northern China, mica-gneisses and granite-gneisses with associated schists may be regarded as Archean.
Their great development in the isle of Lewis has given rise to the term "Lewisian" (Hebridean), by which the gneisses of this region are now generally known.
Although a great area is occupied by crystalline rocks in New Zealand, the Archean age of any portion of the series is not yet satisfactorily established; the lower granites and gneisses may belong to this period.
Sark is composed almost wholly of hornblende-schists and gneisses with hornblendic granite at the north end of the island, in Little Sark and in the middle of Brechou.
Chabazite occurs with other zeolites in the amygdaloidal cavities of basaltic rocks; occasionally it has been found in gneisses and schists.
The parallel banded structures of many granites, which may be original or due to crushing, connect these rocks with the granite gneisses or orthogneisses.
At one time these rocks were regarded as Archean gneisses of a special type.
In the collections made by Watson and Hoadley at the Western Base (Queen Mary Land) gneisses and schists were ascertained to be the predominant types.
The rocks at the summit were granites, gneisses and schists.
The rocks appeared all to be gneisses and schists.
They occur attached to the walls of crevices in the gneisses of the Alps, the Binnenthal near Brieg in canton Valais, Switzerland, being a well-known locality.
The prevailing types are granites, gneisses and schists.
The central zone of crystalline rock consists chiefly of gneisses and schists, but folded within it is a band of Palaeozoic rocks which divides it longitudinally into two parts.
Gneisses and schists of the } Igneous complex of continental platform.
The earliest signs of igneous activity in Africa are to be found in the granites, intrusive into the older rocks of the Cape peninsula, into those of the Transvaal, and into the gneisses and schists of Central Africa.
In Ecuador there is still an inner chain of ancient gneissesand schists and an outer chain composed of Mesozoic beds.
In some gneisses the parallel flakes of mica are scattered through the quartz and felspar; in others these minerals form discrete bands, the quartz and felspar being grouped into lenticles separated by thin films of mica.
Hornblende-gneisses are usually darker in colour and less fissile than mica-gneisses; they contain more plagioclase, less orthoclase and microcline, and more sphene and epidote.
There are also many examples of gneisses of mixed or synthetic origin.
Cordierite-gneisses are a special group of great interest and possessing many peculiarities; they are partly, if not entirely, foliated contact-altered sedimentary rocks.
These sedimentary gneisses (or paragneisses, as they are often called) are often rich in biotite and garnet and may contain kyanite and sillimanite, or less frequently calcite.
In mica-gneisses sillimanite, kyanite, andalusite and garnet may occur.
Hence, though most gneisses are Archean, all gneisses are not necessarily so.
Pyroxene-gneisses are less frequent but occur in many parts of both hemispheres.
The older gneisses of Norway, Sweden, and the Hebrides, of Bavaria and Bohemia, belong to the same age, and it is not unlikely that similar rocks in many other parts of the old continent will be found to be of as great antiquity.
It is highly resistant to weathering and it is doubtless due to its presence in large amount that such comparatively soft rocks as the calc-gneisses take part in forming some of the highest summits.
It is probable that the biotite-gneiss is an igneous rock intrusive in the calc-gneisses and schists, but this and many other puzzling features of the crystallines require more detailed study than I was able to give this year.
Associated with the limestones and calc-gneisses are quartzites and tourmaline-biotite schists which probably represent the lowest portions of the shales immediately overlying the limestones.
They are now fine flaggy micaceous gneisses and mica-schists, which certainly could not have been developed out of any such Archæan (that is Hebridean) gneiss as is now visible to the west.
There is indeed every reason to believe that such gneisses were probably originally true granites, and that their foliation and recrystallization have been the result of metamorphism.
In many places gneisses that possess a thoroughly typical foliation have been found to pierce ancient sedimentary formations as intrusive bosses and veins.
Talc deposits consist of lenses and bands in metamorphic limestones, schists, and gneisses of ancient age.
More sediments are formed than are changed to schists and gneisses, and more schists and gneisses are formed than are changed back to igneous rocks.
In North Carolina and Georgia, the corundum occurs in vein-like bodies at the contact of peridotite with gneisses and schists, and also in part in the peridotite itself.
Ceylon graphite occurs in veins and lenses cutting gneisses and limestones.
Garnets result mainly from contact metamorphism, and commonly occur either in schists and gneisses or in marble.
The above list will hopefully give you a few useful examples demonstrating the appropriate usage of "gneisses" in a variety of sentences. We hope that you will now be able to make sentences using this word.
