564 ERUPTIVE AND METAMORPHIC ROCKS. 



Micas, either Potash Mica (Muscovite) or the Magnesian Micas (Biotite and 

 Phlogopite), and occasionally Lithia Mica (Lepidolite). Minerals of the 

 Amphibole Group, such as Hornblende, Actinolite, Smaragdite, etc. Schorl, 

 Apatite, Sphene, Garnets, Magnetite, Titaniferous-iron, etc., are often present. 

 Epidote, Pyrites, Limonite, Talc, Serpentine, Steatite, Chlorite, Kaolin, etc., 

 etc., which are of frequent occurrence, have mostly been formed subsequently 

 to the consolidation of the rocks in which they are found : they cannot therefore 

 be regarded as normal components of those rocks, except perhaps in a few 

 instances, but have resulted from the alteration of some of the original com- 

 ponents, or from the infiltration of fresh substances in solution. Their secondary 

 origin is most clearly discernible when they occur as pseudomorphs after 

 definitely developed crystals of other minerals. Such pseudomorphs are of 

 frequent occurrence in many eruptive rocks, but are often so minute that in the 

 absence of microscopic examination they would evade detection. 



The minerals that most commonly occur in those eruptive rocks which contain 

 less than 60 per cent of Silica are the following : — 



Plagioclastic Felspars; including Oligoclase, Albite, Anorthite, and Labradorite, 

 all of which crystallize in the triclinic system. Leucite, Nepheline, Augite, 

 Diallage, Hypersthene, Enstatite, Olivine, Hauyne, Nosean, Apatite, Magnesian 

 Micas, Chalybite, Magnetite, Titaniferous-iron, Haematite, Limonite, Pyrites, 

 Oxides of Manganese, such as Pyrolusite, Psilomelane, Wad, etc., Serpentine, 

 Steatite, Greenearth, Chlorite, Calcspar, Aragonite ; Zeolites, such as Mesolite, 

 Stilbite, Heulandite, Analcime, Natrolite, Scolezite, Thompsonite, Phacolite, 

 etc., etc. Chalcedony and Agates frequently form geodes in the vesicular cavities 

 of some volcanic rocks. 



In sedimentary rocks which have undergone subsequent alteration from the 

 contact or proximity of eruptive masses, the most commonly occurring minerals 

 are the following : — 



Garnets, Staurolite, Andalusite, Chiastolite, Kyanite, Chlorite, Mica, Talc, 

 Steatite, Calcspar, Quartz, Magnetite, Pyrites, etc. 



There are some rocks with a schistose structure, and often 

 exhibiting fohation, which it is difficult to regard as eruptive 

 rocks, and to which it is equally difficult in all cases to assign 

 a sedimentary origin, since some are at times seen to graduate into 

 truly eruptive, and others into rocks of an unquestionably sedi- 

 mentary character. And there are other schistose rocks that 

 have no doubt been originally ejected from volcanic vents in the 

 form of fine ashy material and lapilli, which also have a sedi- 

 mentary character due to the assorting and deposition of these 

 materials in waters once adjacent to the seat of volcanic activity : 

 such rocks consequently possess affinities partly to eruptive and 

 partly to sedimentary rocks. 



Hornblende Schist for example comes under the first clause, as it is sometimes 

 seen to pass into Diorite. ' 



Mica Schist may be taken as an example illustrative of the second clause, as it is 

 often seen to pass into ordinary slates ; wiiile some Basalt-Tuffs, etc., may afford 

 fair instances of matter of eruptive origin deposited as a sediment. 



In the present state of knowledge it is convenient to group all the Schistose rocks 

 together, although their origin has, no doubt, in many instances differed very widely. 



The following lists will serve to give some idea of the general 

 mineral composition and physical character of the rocks of most 

 common occurrence, and for convenience they will be arranged 

 in two groups, the Eruptive and Schistose. 



' Mr. J. J. H. Teall has also noted the partial conversion, on a small scale, 

 of dolerite or diabase into hornblende schist in a dyke near Scourie in Sutherland- 

 shire, Q.J. xli. 133. 



