composition, as to be termed, with seeming propriety, a compressed granite.

The greater part of the primary ridge of Cul-coscrehan, which is very remarkable, as occupying a considerable space, within the basaltic and secondary area, is composed of this rock. The section which passes through this mountain deserves peculiar attention, as it discovers the parts in which the transition, from gneis to mica-shist, has been noted; and also where the primary or granular limestone is stratified and intermixed. Gneis of several variteies may, likewise, be found in those ranges of shist mountains, which extend from Loghermore to Sliabh-gallan, especially as they draw nigh to the granitic formation.

Porphyry

Is found in the streams which cut deep channels between the shists and granites of Sliabh-gallan. In such situations, it is seldom wanting; seeming to be, as well as gneis, one of the boundaries from which we find, on one hand, a mutuation into granite, and on the other, a transition into shistus. Immense blocks of this fossil might be found of fine grain, and brilliant colours, yet generally, of reddish hue; fit indeed, for the tool of the sculptor, but too difficult and too expensive to be worked, except for statuary or architectural purposes, and these are now but little thought of in the county of London-Derry.

Granite.

One of the most interesting facts, in the lithology of this county, is the appearance of granite, of all grains, blends, and consistencies, immediately emerging from beneath the stratum of sandstone, which lays below the white coralite limestone and its incumbent basalt, on the northern summit of Sliabh-gallan.

According to the usual circumstances, we should find, between the primitive and secondary rocks, a copious stratification of sandstone,

which, at some considerable interval, is found to rest upon mica-shist, and this again, mutuating into families of silicious shist and porphyry, passes at length into a granitic formation, as we have already observed.

As an exception from this rule, we find, under Cairn-daisy and Tinteagh, in the first instance, granite emerging from beneath a stratification of the usual argillacious red sandstone, scarcely amounting to the depth of one hundred feet.

The angle at which the main couch of the granite bassets, is about twenty degrees S. W.; that of the incumbent red grit is, as usual, about five or six degrees, also S. W. But here I may remark, that there is no analogy between the dips and the directions of the primatives and the secondaries which rest upon them. The map and sections will shew where and with what accompaniments granite may be found.

Primitive or granular Limestone.

This substance is found intimately involved and veined among the primitive rocks, chiefly those of mica-shist. It is generally of a blueish cast, and shines on its surface with mica: not unfrequently it is diversified with cristallised spar of lime, in large veins, and sometimes in parallelopiped figures; it usually contains a considerable proportion of magnesian earth. The blue colour is derived from the iron which it contains: its fracture exhibits a granular arrangement; no shells nor other organised forms are ever found in this fossil; sometimes it abounds so much in silicious earth, as to vitrify when too strong heat is applied in burning it; more frequently it is vitiated with a large proportion of argillacious earth, which greatly impairs its quality as a cement. However, it is very often found so pure, as to form the most durable plaister, and is now extensively used, in the districts adjacent to its quarries, both for building and manure. The character attached to it, in the map and sections, will apprize the reader where this fossil may be found.

Quartz, Flints, and Cristals.

The two former of these fossils are so common, that I shall only make one observation, which relates to them both. The siliceous matter, like the calcareous, has two distinct appearances, which denote the regions, of which they are respectively the natives.

The silica in the shist country, when not blended with the predominant strata, is in the character of quartz, and the lime of the same country is bluish, granular, and laminated. In the basalt countries, the silica is in the character of flint and petrosilex, and the limestone is white and abundant in marine exuviæ. Both quartz and flint are of various tinges; the former sometimes clear, yellow, brownish, reddish, purple, &c.; the latter horn-coloured, purple, brownishblack, &c. Flint has sometimes marine impressions, the quartz none.

As to cristals, these are various. Zeolite in rose-cut surfaces, in points, and in thistle-down, all beautiful, are common in basalt. In granular lime-stone are found white and yellow calcareous spath, in a variety of figures.

In the regions of shist, exclusively, we find the rock cristal. The finest of these are in Muldonach, Mullaghash, and in the other primitive mountains near Learmount. As to size, they are found from the weight of one ounce, to that of twelve. In shape, they are for the most part truncated prisms of six sides and six facets. Those without form are of larger bulk; there is now in possession of Miss Ogilby of N. L. Vady, a mass of quartz cristal weighing not less than seventy pounds.

Secondary Strata.

If we comply with the generally received opinions, we shall be induced to admit, (without much enlisting ourselves in any controversy,) the following geological postulates: first, a pre-existing state, which afforded to the present primitive strata the materials out of which they have been consolidated, according to their actual arrange

M

ment: secondly, a subsequent stratification composed of the impalpable detritus, the crumble and the rubble of the pre-existing stratiform and consolidated rocks; such as we call bolders, landstones, gravels, sands, &c. &c. The order of their deportation and deposition is remarkably distinct; generally speaking, we find, in the lowest situations, and resting immediately upon the primitives, that which is called sandstone.

Sandstone.

Neither the component materials, the angle of inclination, the polarity nor the dip, of this stratified substance, has any reference to that of the former structure, on which it rests; I have observed, indeed, that there is a great tendency to a micaceous surface and form, in such of the sandstones as lay immediately upon primitives of a micaceous character: nor is this strange, considering that, out of the materials furnished from the decomposition of these original rocks, the sandstones may have been furnished with the materials, of which they are an agglutinated recomposition; obeying in their form, the laws which regulated the formations of the primitives themselves, according to the materials which prevailed in their composition.

But, though the sandstone has no reference, as to parallelism, with the sub-strata, it has almost an undeviating arrangement, in this respect, with its intermixed and incumbent accompaniments; that is to say, with the bituminous, calcareous, and basaltic strata. All these we shall briefly describe in their order, remarking only at this place, that the internal divisions of the sandstone, like almost all other works of nature, are not without an effort at regular arrangement; that they chiefly affect the figure of a parallelogram; that they consist of materials sometimes as fine as the most minute sand, and sometimes, even in the same quarry, exhibit a mere congeries of agglutinated paving-stones and pebbles; that sometimes the cement is calcareous; in which case, the stone will frequently effervesce

with an acid; that this cement is more frequently of a flinty character: The former generally hardens on exposure to the air; the latter is hard or soft, in proportion to the prevalence of its cement, and the absence of iron, which metal, according to its various states of chemical combination, acts a wonderful part in forwarding both the structure and decay of organized and natural forms.

The sandstone seems to have remained, for the most part, with its arenaceous particles undissolved. Possibly the fluoric acid has not been abounding, and the water not holding more than 1000th of its own weight of silica in solution, there has not been present any sufficient solvent for so much flinty earth. It would be necessary to investigate much, and to conjecture still more, what were the effective menstrua, what neutralised, what saturated, what precipitated, and what agglutinated; but let us leave the regions of theory and proceed to facts.

Bituminous Substances.

We have very little to do with these matters. Except a few seams (for they amount not to strata) of blind coal and of bituminated shale, there has not hitherto been discovered any thing worthy of remark in this county. At Fahanvale and in Ballynascrene are some indications, which I have marked in the map; wherever any of these appear, in the sandstone itself, or in the marley shale, which is included in its strata, they are always parallel to the general arrangment of the strata.

Secondary Limestone.

Of this there are several varieties. Lowest in stratification is the testaceous variety, which, according to my present view of this matter, extends from the valley of Desertmartin to the flats of Lough Neagh, and is the same which, being intermixed with sandstones, constitutes those extensive couches, passing through parts of the counties of Tyrone and Armagh, to the bases of the primitive mountains, which bound those level countries. A vast portion of the surface of the earth, in secondary and low-laying countries, is formed of this calca