« PreviousContinue »
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.f These classes, however, necessarily merge into one another. Granite may be a true intrusive rock, or may have been formed in situ [i.e. where it is now foundj 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
* 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.
t 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.
due to those rocks being composed of a (suppose siliceous) material not easily affected by metamorphic action.
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 Hybeid Eocks of Durocher, and they include all the rocks originally called Eueytes by jJaubuisson [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
vate rocks as Dolomyte, Ophyte, Steatyte, &c, in which a secondary or pseudomorphic action has taken place—new minerals replacing the old constituents, and thereby changing the nature of the rock.
The second Order (derivate Eocks) may be divided into two classes; namely, I. Subaqueous, II. Subaerial, which need not be dwelt upon now.
This Manual is divided into three parts. Part I. is a classified table of the rocks; Part II. is a description of the Ingenite rocks; and Part III. is a description of the Derivate rocks: while in the Index will be found an alphabetically arranged list of local, duplicate, and other rock names that do not appear in the classified table of the rocks. As before stated, Dana's termination of yte for rook names, on account of its convenience in distinction, is adopted; but at the same time the names ending in ite are also given for such as prefer them. The termination oid is only used to signify like, as Granitoid, granite-like; ous means a constituent that gives a character to' the rock, as Pyritous granite, granite containing pyrite; while the termination ic points to quantity, as folsitic, containing a quantity of felsite (orthoclase intimately combined with quartz.—See quotation from Cotta, page 46).
In Parts II. and III. a general description of each class is given, and a general description of each group; while under each group will be found the subgroups, and under the latter the varieties, and, when necessary, the subvarieties. In these two parts names of classes are printed in large capitals, of groups in egyptian type, of subgroups and particular varieties in small capitals; while the
others, subvarieties and local names, are in italics. Synonymous names for groups, subgroups, and varieties are given when not highly objectionable, but the name that is considered most applicable appears first. Objectionable names, if mentioned, are printed in italics, and the objection to them pointed out.
All the groups are arranged under roman capitals; the subgroups under italic capitals; the varieties -under small italic, and the subvarieties under small Toman letters.
A student wishing to learn the description of a rock, must first look for the name in the Index, and if the name does not occur there, it will be found in the classified list in its proper group, class, and order; he must then refer for the description to either Parts II. or III.
Note.—While this Manual was in the press, Professor Joseph Le Conte's excellent paper on the "Features of the Earth's Surface" was published in "The American Journal of Science and Art," third series, vol. iv. As in it the formation of metamorphic granite is explained (p. 468), the student may be referred to it.