INTRODUCTORY EMARKSON 5 Daubuisson [from eureo, to flow fell for 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 ROCKS) 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 rock 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 felsitic, 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. 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 In these 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 roman 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. UNIVERSITY PART CLASSIFIED TABLE OF THE ROCKS. Order No. I.-INGENITE ROCKS. (Divided into Classes, Groups, Subgroups, and Varieties.) Class I.-GRANITIC ROCKS. Group A.-INTRUSIVE GRANITE (Highly siliceous granite). Subgroup A. Pyritous intrusive granite. Group B. Var. a. Beresyte. B. Albitic granite. C. Schorlaceous granite. D. Granityte. E. Felsitic granite. F. Greissen, or quartzitic granite. H. Pegmatyte. Var. a. Graphic granite. b. Blumen granite. c. Plumose granite. GRANITE FOR THE MOST PART NON-INTRUSIVE (Basic or oligoclase granite). Subgroup A. Hornblendic granite. Var. a. Titanitic granite. b. Chloritic granite. c. Talcose granite. d. Epidotic granite. B. Graphitic granite. C. Hematitic granite. D. Pyritous oligoclase granite. |