torn away portions both of the harder and softer beds; and the disintegration of the granitic aiguilles which are exposed to the influence of atmospheric agency is daily taking place, and their ruins are every day falling on the surface of the glaciers, and are carried down into the valleys: their peculiar forms are derived from their laminated structure, which disposes them to split in a vertical direction*.

* Plate 2. fig. 2. represents the general position of the beds near the Col de Balme, and Mont Blanc; a a a alternating beds of sandstone and limestone; bb elevated beds of puddingstone, containing rounded stones and fragments of the lower rocks; c c soft slate, in which a passage or Col is formed; d d d vertical granitic beds rising in pyramidal forms, called Aiguilles or Needles.

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CHAPTER V.

ON ROCKS GENERALLY DENOMINATED PRI

MARY.

Classification of Primary Rocks.-Granite; its constituent Minerals. Varieties of Granite.-Structure of Granitic Rocks.General Appearance of Granitic Mountains.-Granitic Aiguilles. Structure of Mont Blanc.-Principal Localities of Granite.--Situations in England where Granite is found.Granite Veins. On what has been denominated Secondary Granite. On the Passage of Granite into Felspar-porphyry and Sienite.-Minerals that occur in Granite; Uses to which it is applied.

IF

any rocks can with propriety be denominated Primary or Primitive, they are those which are most widely spread over the globe in the lowest relative situation, and which contain no remains of organic existence. Primary rocks are supposed by Geologists to constitute the foundation on which rocks of all the other classes are laid; and if we take an enlarged view of the structure of the globe, we may admit this to be the fact,-but the admission requires certain limitations. The same causes that have produced granite and the other primary rocks in immense masses below all other rocks, have in some situations reproduced them in smaller masses, covering rocks belonging to the Transition or Secondary Classes. No systems of classification can be made so definite in the mineral kingdom, as those which relate to organic beings in the vegetable or animal kingdoms. It will however be convenient to retain the term Primary Rocks, when treating of

those rocks which, according to the present state of knowledge, are the foundation rocks of every country, although some of these rocks may occasionally make their appearance in the upper formations.

Primary rocks are chiefly composed of the hard minerals, quartz, felspar, and hornblende; the minerals, mica and talc, are disseminated in smaller proportions, and limestone and serpentine occur in beds or masses, but less frequently than the abovenamed minerals. If we refer the slate rocks to the Transition Class, the few simple minerals here enumerated constitute nearly the whole of the mountains denominated Primary.

The structure of primary rocks is crystalline; they form the central parts of the most elevated mountain chains, and they occur at the lowest depths that have yet been explored, and are hence believed to be the most ancient of rock formations.

Werner has enumerated fourteen primary rocks : but as some of these have only been found hitherto in one place, it appears improper to consider them as distinct orders, unless we arrange every variety of rock in the same manner, and increase the number of orders indefinitely*.

*The system of classification introduced by Werner, was formed principally from observations made in Saxony, and had great merit, as illustrating the geology of that part of Germany: but it has been objected with much reason to the general adoption of the terms he employs, that they were framed to suit a particular theory, before a sufficient number of facts had been collected to warrant its reception. Subsequent discoveries have also proved, that the different classes into which Werner has divided rocks, have not the marked and definite characters necessary to constitute a natural system of arrangement.

The following arrangement of Primary Rocks will, I trust, be found both simple and intelligible, and as conformable to nature as the present state of our information will admit. It includes only three principal rocks as primary; granite, with gneiss and mica slate, which are nearly allied to granite, and form an incrustation over it: these never contain organic remains, and they have rarely been observed lying over other rocks in which such remains are found.

CLASS I.

Principal Rocks denominated Primary.

1. Granite, comprising all the varieties of this rock, and small-grained granite passing into porphyry. Eurite of the French Geologists, primitive porphyry of the Germans.

2. Gneiss or Slaty Granite.

3. Mica Slate.

Subordinate Rocks which occur among Primary.

Hornblende Rock.

Serpentine.

Crystalline Limestone.
Quartz Rock.

Some of these subordinate rocks occur also among rocks of the Transition Class *.

* Since the publication of the first edition of this work, M. Brongniart, a celebrated French mineralogist, has proposed an arrangement in many respects similar to what I had adopted. The characters of Brongniart's first class agree with those I had before given in this work. "La première classe renfermerait les terreins dans lesquelles on n'a encore découvert aucun débris de corps organisés dont la structure est crystallisée, et dans la composition desquelles les roches granitiques proprement dites sont dominantes."-Journal des Mines, Mai 1814.

The three principal rocks of the Primary Class, -granite, gneiss, and mica slate,-might with propriety be regarded as belonging to one formation. They are essentially composed of the same minerals varying in different proportions, and are rather modes of the same rock, than different species. They pass by gradation into each other, as one or other of their constituent minerals become more or less abundant; they alternate with each other in various situations, and may be regarded as cotemporaneous. It may, however, for the convenience of description, be proper to treat of each separately.

Rocks of the First Class.

Granite is considered as the foundation rock on which slate rocks and all secondary rocks are laid. From its great relative depth, granite is not frequently met with, except in alpine situations, where it appears to have been forced through the more superficial covering of the globe. Where granite rises above the surface, the beds of other rocks in the same district generally rise towards it, and their angles of elevation increase as they approach nearer to it. Some writers derive the name from geranites, a word used by Pliny to denote a particular kind of stone; others, with more probability, suppose that the name originated from its granular structure, or the grains of which it is composed. Granite is composed of the three minerals described in the third chapter,-quartz, felspar, and mica; which are more or less perfectly crystallized, and closely united together.

The three minerals of which granite is composed vary much in their proportions in different granitic