The first letter is silent and the two vowels rhyme with eye which means that the proper pronunciation of gneiss is nice. The gneisses, and there are many of them, are very common rough and ready rocks. Lumps and layers of them squat in and on the earths crust as though they had been doing nothing else for millions of years. This may well be true, for a, few million years is but a sentence in the age long diary of the earth’s minerals.
The countless different rocks of the earths crust fall into three general classes, depending upon how they were formed. The original rocks were formed perhaps four billion years ago and their life story is an endless warfare with weather and water, volcanic fury and the heaving up and down of the earth. The sedimentary rocks, such as clay and slate, were formed under water from silty sediments. The igneous rocks, such as lava, were formed by seething volcanoes. But the steady and relentless warfare continues and certain sedimentary and igneous rocks were further changed to form the third class of metamorphic rocks.
The gneisses are metamorphic rocks and the long story of their age old adventures is often written on their rough faces. They are coarse and usually bumpy rocks of speckled and freckled colors ranging through whites and greys, tans and browns and maybe dusty tones of green and pink. The colors in
The colors in these dense and durable rocks tend to form irregular stripes and bands, a feature which sets the gneissos apart from other similar metamorphic rocks
The colors and bumpy granules in a lump of gneiss are fragments of the parent rocks from which it was made. Some samples show flattened pebbles pressed and cemented in a bed of different rock, others show flat lenses of mica folded in bands of heavy quartz and feldspar. These features of the gneisses were formed when granites, gravels, quartz and feldspar were buried deep under the terrific heat and pressure of the earth's crust. The varieties are named either for their mineral content ol; their markings. Muscovite gneiss glistens with lenses of silky mica, foliated gneiss is banded with leafy layers.
The gneisses make sturdy building stones. However, they tend to cut in one direction along with their bands which means that they cannot be shaped as well as granite. Many types of gneiss are found in New England and Canada, around Lake Superior and in all of the western mountains of North America, There is no shortage of these sturdy rocks which tell us so much about the long, tortuDus history of the earth's crust: