Igneous rocks in great variety and in all forms incident to an extruded or volcanic and intruded or plutonic origin are present.
In the cooling and crystallizing of igneous rocks from a state of fusion many minerals are formed, the most common being silicates of the alkaline earths, which are usually inclosed in a glassy or cryptocrystalline base.
The rocks exposed in the mountains are largely of sedimentary origin, but a characteristic feature is the geology, especially to the west, in the presence of extensive volcanic areas and lofty mountains of igneous rocks.
Volcanic ashes are sometimes represented by dots, at other times by signs somewhat similar to those which are used for true igneous rocks.
Igneous rocks have, in many cases, forced their way up through stratified rocks.
Acmite is a characteristic constituent ofigneous rocks rich in soda, such as nepheline-syenites, phonolites, &c.
Aenigmatite and its variety cossyrite are rare minerals forming constituents of igneous rocks of the nepheline-syenite and phonolite groups.
A group, whether of alluvial deposits, sedimentary strata, or igneous rocks, referred to a common origin or period.
As every one knows, there exist in this and many other countries enormous masses of igneous rocks, which have certainly been extruded from below.
Of course, plateaux of accumulation are not always formed of igneous rocks.
Those geologists who deny the existence of this system consider these rocks as parts of the silurian system which have been most disturbed by subterranean forces, and most altered by proximity to igneous rocks.
The form in which the igneous rocks, of which the entire crust of the earth was originally composed, now appear, furnishes no assistance in judging of the amount of denudation which they have suffered.
They are considered by perhaps the majority of the geologists who have studied them most carefully to be igneous rocks intruded in a molten state among the sedimentary rocks involved with them.
Dikes of igneous rocks may be followed into a region where they have been foliated by pressure.
The most thoroughly metamorphosed rocks may sometimes be traced out into unaltered sedimentary or igneous rocks, or among them may be found patches of little change where their history maybe read.
This familiar rock may be studied as an example of the second great group of rocks,--the unstratified, or igneous rocks.
Lava, basalt, and obsidian are characteristic types of this group of igneous rocks.
The zeolites occur very commonly in cavities in igneous rocks, and also in mineral veins.
Diagrammatic vertical or structure section through a portion of the earth illustrating the common modes of occurrence of igneous rocks.
The feldspars occur in all three great groups of rocks, but they have most commonly crystallized during the cooling of molten masses of igneous rocks.
This familiar rock may be studied as an example of the second great group of rocks,--the unstratified, or igneous rocks.
The above list will hopefully provide you with a few useful examples demonstrating the appropriate usage of "igneous rocks" in a variety of sentences. We hope that you will now be able to make sentences using this group of words.
