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HANDY-BOOK OF ROCK NAMES.
MHE term Rock, in a geological sense, includes
I every solid substance that is an ingredient, or forms part, of the earth. Thus loose sand, clay, peat, and even vegetable mould, geologically speaking, are rocks. Jukes thus defines a rock :-"A mass of mineral matter consisting of many individual particles, either of one species of mineral, or of two or more species of minerals, or of fragments of such particles. These particles need not at all resemble each other in size, form, or composition; while, neither in its minute particles, nor in the external shape of the mass, need a rock have any regular symmetry of form.” Rocks are most variable in condition and structure; soft or hard, loose or compact, friable or tenacious, coarse or fine, crystalline or homogeneous; or they may be scoriaceous, vesicular, hyaline, &c. &c.
Rocks may be chemically, mechanically, or organically formed, or two or more of these combined; they may be stratified or unstratified, igneous or aqueous, or partaking of the nature of both. Various classifications have been adopted by different writers on the subject, each taking dif
ferent peculiarities as a foundation for his system. Jukes and others have divided rocks into four classes; namely, Igneous, Aqueous, Aerial, and Metamorphic; while Forbes, who wrote subsequently, has simplified this division, and makes two great classes of all rocks.
Forbes calls his first class by the names INGENITE or SUBNATE Rocks; i.e., “such as are born, bred, or created within or below;" and the second he calls DERIVATE Rocks, “since directly or indirectly they are all derived from the destruction of the former.”
Under Ingenite rocks are included all the true igneous, intrusive or irruptive rocks, whether they are still in their original state, or whether they have been subsequently affected by metamorphic action, as also the metamorphosed sedimentary rocks; since all these have been bred or formed within or below the surface of the earth. Thus all granites, whether truly igneous or metamorphic,* are included. The Derivate order consists chiefly of sedimentary rocks, but it will include some, such as DOLOMYTE,F HALYTE, &c., which some authorities refuse to regard as sedimentary rocks.
In this Manual these suggestions will be followed, and the rocks classed in two orders; namely,
, * Some authorities deny that any granite can be of metamorphic origin. To me, however, there does not appear to be any room for doubt, for in different places I have found granite graduating through gneiss and schist into unaltered rocks.
+ Dana suggests that all rock names ending in ite should be changed into yte, and the first termination should be kept solely for minerals. A general adoption of this suggestion would prevent the confusion that at present exists when so many rock masses and minerals have similar names,—such as AUGITE, STEMME, &c. &c. Dana's names for the minerals are those
used in this Manual.
Order I., INGENITE Rocks; and Order II., DERIVATE Rocks.*
The first Order (INGENITE Rocks) can conveniently be divided into four classes; namely, I. GRANITIC, II. PLUTONIC, III. VOLCANIC, and IV. TRANSITION, or METAMORPHIC SEDIMENTARY Rocks.t These classes, however, necessarily merge into one another. GRANITE may be a true intrusive rock, or may have been formed in situ fi.e, where it is now found by extreme metamorphic action. Granite is supposed to have been formed under great pressure, at a considerable depth beneath the surface of the earth; consequently it is never accompanied by tuff. Of it Jukes says :-“ As the granite rocks are all hypogenous, or nether-formed that is, have all been consolidated before reaching the surface of the earth, they are necessarily devoid of 'ash,' [tuff,] or of any mechanically derived accompaniment whatever."
Usually the sedimentary rocks associated with granite are more or less metamorphosed : sometimes the metamorphism is very slight indeed. This may be due to the granite having been formed at a lower zone or depth, and intruded into its present place in a semi-liquid state, under such low pressure that its heat, latent or otherwise, was unable to affect the associated rocks; or it may be due to those rocks being composed of a (suppose siliceous) material not easily affected by metamorphic action.
* Sterry Hunt and others have called the Derivate rocks by the name of INDIGENOUS Rocks, and the Granitic and Igneous rocks, Exotic ROCKS. This classification, however, does not appear to include the metamorphic rocks, as they are in part indigenous, and in part exotic.
+ Some of the metamorphic rocks were originally sedimentary, while others evidently were igneous. In this Manual it has been considered expedient to describe the latter with the groups to which they originally belonged.
Allied with the granites are granitic rocks, in which a considerable portion of the quartz crystallized out prior to the other constituents. This, according to Scheerer and others, debars them from being classed as true granites. Nevertheless, this definition is controverted by Forbes and others, who contend that in some true granites a portion of the quartz crystallized out previous to the other constituents. These granitic rocks or ELVANYTES (Quartz-porphyries) are allied to the granite, being mineralogically similar, and always destitute of tuffs : they never occur contemporaneous or interstratified with the sedimentary rocks, but rather in irruptive masses or dykes; moreover, in places they merge into granite. On the other hand, it is not unusual at the margin of a mass of Elvanyte, or in dykes branching from a mass, to find Elvanyte changing into a rock identical with a Felstone. From these facts it would appear that Elvanyte is a connecting link between Granite and the Plutonic rocks. There is also another somewhat similar link; namely, the metamorphosed Plutonic rocks, as they merge into Granite.
The rocks here called PLUTONIC rocks can be divided into two groups; namely, FELSTONES, or highly siliceous rocks, and WHINSTONES, or basic rocks. Some of the rocks placed in the first of these groups are however a connecting link between the two, partaking of the nature of both, and merging on the one hand into Felstone, and on the other into Whinstone. These basic felstones are the HYBRID ROCKS of Durocher, and they include all the rocks originally called EURYTES by Daubuisson (from eureo, to flow well or easily], on account of their fusibility. Felstones proper contain over 68 per cent. of silica; Durocher's Hybrid rocks between 65 and 55 per cent.; while the Whinstones contain a large percentage of pyroxene or amphibole, or such minerals.
Any of the Plutonic rocks may be contemporaneous, i.e. interstratified with the sedimentary rocks. Under such circumstances they are often associated with beds of tuff. Tuff also on some occasions occurs in dykes and pipes, when, apparently, it is the remains of old vents or funnels of eruption.
The VOLCANIC rocks appear capable of a division similar to the Plutonic rocks; the highly siliceous or the TRACHYTES, and the basic, or the AUGYTES. In them are also hybrid rocks, for which Scrope has proposed the name of GREYSTONE, and Abich that of TRACHYDOLERYTE. Volcanic rocks generally are accompanied by tuffs ; of which more hereafter.
The METAMORPHIC SEDIMENTARY Rocks may be placed here. They belong, indeed, also to the Derivate Order; but, being compelled to make a selection, we place them under the Ingenite, as this arrangement seems to involve the least inconvenience. They are Ingenite Rocks, as they have been in a certain sense formed below, yet their materials were previously derived from the destruction of other rocks, which gives them their relation to the second Order. They are capable of a nearly inexhaustible subdivision, not only on account of the different degrees of metamorphism to which they have been subjected, but also on account of the numerous and variable constituents entering into them.
Allied to the metamorphic rocks are such deri