LECTURE V.

Primary Rocks--Granite-Gneiss-Mica Slate--Argillite-Serpentine--Limestone--Quartz rockChlorite Schist--Talcose Schist-Hornblende rocksActynolite Schist-Porphyry-Syenite. Identity of formations-Isochronism—Alternation-Loxodromism--Character of Primary Soils.

All geologists agree in assigning an extreme date to GRANITE-and it is usually placed as the first, or lowest of primary, and therefore of all rocks. It is considered as unstratified: is one of the most abundant, and most useful of rocks. It derives its name from its usual granular appearance.

It is placed in irregular masses beneath all other strata, occasionally sending veins into the adjacent rocks. It seldom presents a definite form: but is occasionally so divided by fissures as to make it bear some analogy to stratification. When these crevices or fissures are so placed as to produce a prismatic figure, the angles are sometimes rounded off, and an irregular spheroid is the result.

Granite sometimes presented a minute, but irregular prismatic structure, independant of the above.

It is also occasionally minutely laminar, or exfoliates in crusts, that are sometimes concentric, sometimes flat; in some cases appearing the result of a concretionary structure in the rocks; in others as the consequence of atmospheric action.

The component parts of Granite are Quartz Mica and Felspar: of these three substances I shall give a brief notice-not intending it however as a precedent to be followed in describing the other rocks: but to show the necessity for an intimate acquaintance with mineralogy, previous to commencing the study of Geology. It will also show the connection between mineralogy and che. mistry.

Quartz is a siliceous earth very plentifully distributed. It is frequently limpid, but occurs of all the tints of yellow, green, and red, derived from metallic agents.

Its usual form is a 6 sided prism, terminated by 6 sided pyramids, more or less regular. Not unfrequently it has metallic fibres running through it, and then becoming more valuable, is cut and set as an ornament. It has a great comparative hardness, scratches and cuts glass; and is not scratched by steel. Before the compound blowpipe it melts instantly.

Quartz is the purest variety of siliceous earth, containing about 69, and some even 96 per cent. of pure silex. Rock crystal is quartz. It forms a large proportion in the composition of calcedony, agates, flints, jaspers &c. It is also a constituent in many gems: Opal and Cairngoram are nearly pure quartz. Topaz, Hyacinth, schorl, and tourmaline, aventurine, emerald, beryl and garnet all contain large portions of this earth. It is an important article in the arts; is used in the manufacture of glass, is an ingredient in the preparation of porcelain, and earthenware, and of smalt. It is used in the form of sand, in all mortars-and in agriculture, for the improvement of certain soils.

Felspar is also widely distributed. It is a compound substance, in which silliceous earth is in the greatest proportion, frequently coloured by oxide of iron. The following is the composition according to

It is softer than quartz, and easily fusible. It is usually red, grey, or white, with varieties. The crystals are 4 and 6 sided prisms, bevelled, the primitive form being a slightly obtuse rhomboid.

The extreme beauty of some varieties of feldspar has occasioned its use in ornamental jewelry. Persia, Arabia and Ceylon furnish the green variety, which is much prized. On the coast of Labrador it occurs very beautiful. To Dr. Bigsby, we are indebted for the knowledge of an extensive locality in the vicinity of Lake Huron.

Under the name of Petunze, it is used in the manufactory of porcelain-It was first employed by the Chinese :-is now extensively used by the French at the celebrated Sévres, under the name of decomposed feldspar-and also by the English, who have a valuable locality of it in Cornwall. It retains its whiteness remarkably pure, from there being no iron in its composition. By analysis it yielded according to

The other ingredient usually noticed is Micaa compound mineral, consisting of the earths, silex, alumine, and magnesia, tinged with iron. Its color is grey, passing into brown and black-very rarely, green and rose coloured. It is easily fusible by the blowpipe. Its crystal is a rhomboidal prism; -texture lamellar; It is easily divisible into extremely thin plates. It is scratched by the finger nail. It yields by analysis

In Siberia and Russia this mineral is reguarly mined and is an article of commerce, under the name of Muscovy glass. Its flexibility has caused its use instead of glass, in the Russian ships of war as it is not liable to breakage by the discharge of guns. Considerable quantities are exported :200 puds being yearly sent to Lubec, and a large quantity to England and Ireland.

These three substances, quartz, felspar and mica, are usually considered as the component parts of Granite, and so they are, generally. But it is a distinction not sufficiently correct for geological