MRS. L.-It was there placed, under the name of Quartz Rock, (Fig. 2., Plate 5,) when we were speaking of the constituent parts of Granite. I there told you it was found in separate masses of great extent, and described its properties and appearance-volume 5, page 338. I now allude to it again, as one of the Primary Rocks, and will name whatever else is known of it. You must often be content to hear of the same substance twice-as most of these rocks, &c., appear again in altered situations and conditions.

MAT. That will be rather to our advantage, provided we can keep our ideas of them distinct, and have a right understanding of the cause of the repetition.

MRS. L.-This we will endeavour-and for the purpose, put out of mind now the beautiful Rock Crystal I have shewn you as pure Quartz; and do not, like a girl I once taught, fancy the Dublin sugar-loaf mountains as transparent as her ear-rings, because I told her they were composed of Quartz. Quartz Rock consists either of pure Quartz, compact or imperfectly granular; or it is a compact granular compound of Quartz and Felspar, or of Quartz and Mica, subject to other accidents. The obviously re-united structure even of the most compact kinds, together with the marks of stratification, prevent the compounds of Quartz and Felspar from being confounded with Granite, except, perhaps, at the points of junction with that rock. From certain varieties of Micaceous Schiste, it can only be distinguished by the predominance of the Quartz: the limits of the two being evanescent. Quartz Rocks vary exceedingly in dimensions, even from an inch to many yards in thickness; and as they often possess natural joints, they break, like the Schistose Rocks, into rhomboidal or rectangular fragments. The Strata are occasionally, but rarely bent, and never present the contortions so common in Micaceous Schiste. You remember, I hope, what that term means?

ANNE. Perfectly-Slate containing a good deal of

Mica-and I was turning over the leaves to Plate 3, to see what was meant by rhomboidal.

MRS. L.-I did not think you would have forgotten -I never pass a term unexplained, unless it has occurred before: therefore when memory fails, you do right to look back on former conversations-you will be sure to find what you want.

"With regard to the precise place which Quartz Rock occupies among the Primary Strata, nothing positive can be laid down; since it is found alternating with all those which follow Granite. It alternates, that is, lies in alternate beds, sometimes with Gneiss, sometimes with Micaceous Schiste, and by a little change becomes so like them, that it may be scarcely distinguished. Such varieties holding an intermediate state between two rocks, have an equal claim to be ranked under either; but as it is a fruitless attempt to define that to which nature has not set bounds, and as no advantages are gained by multiplying terms to express such gradations, it is most proper to class the doubtful specimen in that division which is the predominant one. But the alternations between Micaceous Slate and Quartz Rock, are often more decided and on a larger scale; a sudden and complete change taking place where they meet. Scarba and Jura, and many other parts of Scotland, afford specimens of this. There are beside, large tracts of country, exhibiting Strata of some thousand feet in thickness, consisting solely of Quartz Rock; giving it thus a decided claim to be ranked as a principal member of the Primitive Class. The varieties in the mineral character of Quartz Rocks are numerous. It is occasionally, but rarely, in a compact state and crystalline throughout. More generally, when pure, it has an aspect obscurely granular, by degrees becoming lax and arenaceous (sandy); as the grains vary in size and in the closeness of their union. The next, and perhaps prevailing variety consists of a mixture of Quartz and Felspar; and from the latter it

generally derives a reddish hue. Its texture is various, the grains of Felspar being at times imbedded in a crystalline Quartz, while more frequently, the whole rock is a mass of agglutinated grains of the two substances, in various proportions. In every instance it has a foliated disposition (a disposition to spilt into leaves.) Another variety is Quartz mixed with Mica. Sometimes the grains become so large as to appear like agglutinations of gravel; nor is it uncommon to find specimens containing rounded pebbles of large size. From these the passage is easy into common Grauwacke, which I have already described to you.

Quartz Rock never appears to contain any essential minerals, capable of changing its general characters; and it is equally remarkable that it is very rarely the repository of any independent mineral, except Garnets and Pyrites. The colours are extremely limited. In the purer varieties, white is the predominant hue; but they are occasionally to be found tinged with ochre yellow, or with reddish, and even dark purple tints. Where Mica is present, the colour is often grey; and the presence of Felspar, in the same manner, often confers a pink or flesh-coloured hue to the compound rock. Fig. 3. is a common kind of Quartz Rock, something different in colour to our former specimen.

MAT.-May I interrupt you to ask what Syenite is. I heard the term used in speaking of the Primitive Rocks, and did not recollect your having referred to it in any way.

MRS. L.-Syenite is not very abundant, and is frequently confounded with Granite. It is in fact little else than Granite with a predominant quantity of Hornblende. Granite Rocks, as I have before explained to you, often contain a considerable quantity of Hornblende-these aggregates are termed Syenites, or Syenitic Rocks, and are of various hues, according as one or other of the constituents predominates. Sometimes the place of the Quartz is wholly occupied by