I. CLEAVED or SLATY FELSTONE.-With cleavageplanes, sometimes so distinct that the rock splits into slates, but rarely into "Roofing slate."

J. FOLIATED or SCHISTOID FELSTONE; SCHISTOSE FELSTONE. A fissile felspathic rock, composed of leaves of felspar, with mica or amphibole, or ripidolite; sometimes with a little quartz. NOTE.-Rocks very similar both in aspect and in composition are found among the metamorphic igneous rocks. Nevertheless it may be possible that some felstones are so constituted that on solidification their constituents arrange themselves in leaves. At Barna, co. Galway, Ireland, breaking up through the porphyritic oligoclase granite, are dykes belonging to this subgroup that apparently are newer than the time at which the general metamorphism of the associated rocks took place.

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K. GRANULYTE, or GRANULITE. "A fine-grained to compact fissile compound of felspar and quartz, usually with some mica."-Cotta.

NOTE. Granulyte, although allowed to remain provisionally among the Felstones, may properly belong to the metamorphic igneous rocks, and be a felstone into which a fissile structure has been introduced by metamorphism. Some of the metamorphic igneous rocks hereafter to be described seem to be identical with the granulite of Cotta.

L. FELSTONE or FELSYTE PORPHYRY; PORPHYRY; ORTHOPHYRE. Felstone, or Felsyte, containing distinct and numerous crystals of one or more felspars.

a. Amygdaloidal (Gr. amygdalon, an almond); b. Vesicular.

If the felsitic or felspathic matrix is full of cavities, the rock is called vesicular, while, if it contains crystalline secretions filling the original cavities, it is called amygdaloidul.

Felstones, or Felsytes, supposed to be metamorphosed.

Metamorphic action as yet has not been properly explained or understood. It cannot be denied that derivate rocks may be altered into schist and gneiss; yet many authorities seem to doubt that igneous rocks could be altered by the action that metamorphosed the others. That the latter have sometimes been altered, seems not only probable but also absolutely necessary, for the following reasons:

There are derivate rocks of all ages, which have their associated igneous rocks. Such derivate rocks, except, however, those of the most recent age, are often metamorphosed; the metamorphism being clearly normal (to use Delesse's expression), i.e. not the result of the intrusion of the igneous rocks, although, as is often evident, effected after that intrusion. The agents which have prevailed to metamorphose the containing derivate rocks must unquestionably have attacked the contained igneous rocks, and that they have sometimes succeeded in altering them also more or less, seems clear from the circumstance, that the igneous inliers just mentioned are often different from any of those that occur in unmetamorphosed sedimentary rocks; which circumstance cannot be regarded as merely accidental.

In some localities, such as West Galway and Mayo, Ireland, there appear to be data for the above suggestions. In these places it has been observed that associated derivate and igneous rocks appear to have been acted on in similar ratio, as expressed in the following table: *

*This table only records changes that have been observed, not, however, all the changes rocks are capable of. The first stages of metamorphism of the Whinstones are so various, that it is hard to give a general name for the rocks.

M. GNEISSYTE or GNEISSITE; GNEISSOID FELSTONE.-A crystalline fissile aggregate of quartz, felspar, and mica, more or less foliated; always occurring in dykes or intrusive masses, associated with gneiss or schist.

NOTE. Apparently this rock is the same as the rock described by Cotta, and called Red Gneiss.

Gneissyte generally does not occur among sedimentary rocks in the first stage of metamorphism * (Schist series), but among highly metamorphosed sedimentary rocks (Gneiss series); dykes and masses of it are often found.

Varieties in Composition.

a. Quartzose Gneissyte; b. Felsitic; c. Micaceous ; d. Hornblendic; e. Syenyte Gneissyte; f. Chloritic or Ripidolitic; g. Talcose Gneissyte. Varieties d, e, f, and g are probably metamorphosed eurytes. The difference between d and e is, that the former contains mica, in addition to amphibole, while in the latter there is only amphibole.

Structural Varieties.

h. Porphyritic, and i. Schistoid.

The latter variety splits easily along the planes of foliation, on account of one of the constituents, usually the mica or the quartz, being only very sparingly developed. It is the passage rock between gneissyte and schistoid felstone.

N. SCHISTOID FELSTONE or FELSYTE.-A fissile aggregate of felspar and mica; generally a little

*For the degrees of metamorphism of the derivate rocks see pages 37 and 76.

quartz is present, but in such small quantities as to seem more an accessory than an essential. The rock mass is more or less foliated. Some schistoid felstones have an unctuous feel, and change by pseudomorphic action into steatyte schist, or even steatyte.

NOTE. The foliation in the metamorphic felstones seems to follow conspicuous structures in the original rocks. Some normal felstones are amorphous, others have a spheroidal or platy structure, while some are cleaved. In the amorphous felstones there are often lines apparently due to viscid fusion, while in the platy felstones the lines are parallel, or nearly so, and at other times oblique to the walls of the dykes. In the metamorphic felstones very similar structural peculiarities can be found; in some there is little or no foliation, but the rocks are banded (see Granitic Felstone, page 35), while in some there is wellmarked foliation, either parallel or oblique to the walls of the dykes, or spheroidal.

Schistoid felstones may be named after conspicuous minerals locally essential; they may be quartzose, or felspathic, or micaceous, or hornblendic, or steatitic, &c. &c.

F. Whinstone;* Greenstone.-A crystalline aggregate of felspar with either pyroxene, amphibole, or uralite. With these minerals, in forming subgroups and varieties, are associated ripidolite, mica, pyrite, marcasite. Under the group of Whinstone are

* The term Whinstone has been adopted in place of Greenstone, as the latter has different significations, according to the authority by which it is used. Jukes and many other British geologists include under that name not only the basic plutonic rocks, but also all the basic felstone, especially when of a green colour. Cotta includes under this name Dioryte and Diabase. Brongniart confines the term to Dioryte, while Naumann restricts it to Diabase. Whinstone seems to be a nearly universal name in England, Ireland, and Scotland, the only objection to it being that in places it is also applied to the basic volcanic rocks.