GEOLOGICAL OBSERVATIONS.

BEFORE EFORE that we enter upon a description of the soils or surfaces of this county, it seems to be of great utility, that we should understand somewhat of the nature of those substances, from which all soils are principally derived, and of which also they are chiefly composed.

The substances which we now proceed to take into consideration, are the solid masses, comprehended under the usual names of rocks. I do not intend to enlarge unnecessarily on this part of the inquiry, still less is it my purpose to involve the reader in intricate speculations. The present enquiry shall be confined to objects whose explanation will materially conduce to the better understanding of other subjects which are suited to the purposes of this memoir.

The first great distinction of those rocks composing the solid masses of this and all other portions of the earth, is derived from the opinions entertained, as to the date of their formation. Under this distinction, they have been divided into primitive and secondary. The primitive are those in which no trace of organised forms, either animal or vegetable, have been found; of this class the most common are those called granite, gneis, shistus, and granular limestone. The secondary are those in which have been occasionally discovered the remains of substances formerly organised; and hence the reason of their distinction; the primitive rocks being considered as having assumed their solid form, before the existence of plants and animals; whilst the latter are accounted to have consolidated their structure, since the multiplied existences of those organised forms, whose exuviæ they are found to contain. The most abundant examples of

secondary strata, are those named basalt, tráppe, growake, chalk, coralite limestone and sandstone, with their accompanying relatives. In examining the sections annexed to the map, we shall see what portions of the surface and solids of this county, are severally composed of each of these distinctive rocks; and in the map itself, the same may be perceived by attending to the mineralogical characters, together with their explanations, in the geological

index.

So far premised, I shall proceed, with some degree of minuteness, to describe what nature has to offer, in this department of her fossil history, within this county.

The river Roe divides the county of London-Derry into two districts, nearly equal but totally distinct. On the west bank of this river lays the territory of primitive rocks, and on the east bank, with but few exceptions, we find an area of secondary formations. Each of these distinctions includes a variety of accompanying fossils: we shall endeavour to enumerate them in their order.

In remounting the course of the river Roe shortly above Daiseyhill, we find in the channel a species of flagstone, which is known to lithologists under the name of mica-shist; of this species of primitive rock consist all the masses which jut over and confine this river at the beautiful cataract of the Dog-leap. This species of rock is greatly intermixed with white quartz, changes often, in the same quarry, into gneis, flag, slate, with other varieties: it is the chief component of Loghermore, and the other mountains of the same district; it is known by its surface, frequently inclined to the colour of gold, by its waving texture, and its smooth, soft feel. Of this the mountain of Mullagh-bhui is composed, though now and then it offers a lamellated structure, imperfectly approaching to slate. Some specimens are gritty, in comparison with others which are of a silky softness.

Although it is obvious that the beds (henceforth for sake of distinction may I be permitted to say, the couches) of shists as well as of

all other primitives, seldom coincide either with an horizontal or a vertical line, yet is it hitherto difficult to find any well authenticated facts, relative to the dips and directions of their strata, which might warrant a general inference. Indeed there is, at first sight, an evident confusion in this respest; nevertheless it has appeared to me, after many observations carefully noted, at different times, and in various districts, that the main beds or couches of these primitives do generally dip toward the north and the east. The various lines of division, discoverable in almost every couch, are among the causes which perplex our observations, and therefore, as these lines have not hitherto been noticed, so as to become reducible to any theory or nomenclature, I shall now venture to make some distinctions intended to throw light on this subject.

The great divisional lines which separate any one of the couches, or strata, from those either above or below, I shall beg leave to name, for the sake of precision, the stratal line: the line which divides the couch, in direction parallel to the mean dip and elevation, and which, when the couch is broken up (as in quarrying) seems like a wall, though seldom quite perpendicular, between the upper and lower stratal lines, I shall beg leave to distinguish as the parietal line. The third line of division observable in most quarries, is that which traverses the mean couch, in various angles (seldom however at right angles) either to the stratal or parietal lines; and this may be termed the transversal line. These distinctions I beg leave to offer as initiatory to a new class of observable phenomena.

Laminated Shist (Flag.)

This, though not so common as mica-shist, is not difficult to be found. The finest quarry I have yet seen, is near the commencement of the new flat road, from Derry to Newtown, under the Mass-house. There seems to be a law of nature, according to which this fossil never coincides, either with a perpendicular, or an horizontal line. I

have remarked that there is a double inclination in the plane of the main couch of pure flag: in one direction, it dips toward the north, in an angle of ten degrees; and in another direction, toward the east, in an angle of five degrees. If you suppose the stratal line produced through the couch, from one parietal plane to its opposite, and cutting, by its extremities, the two opposite transversal planes, in the angle of the eastern dip; the coincidence of these two planes will give the plane of the couch; and the diagonal of this plane will give the sum of its double inclination. The transversal line is the real boundary of the plane or table which is called the flag; and it is bounded on each side by its lateral planes; and above and below it is bounded by the stratal planes. On account of these intersections, the table of the flag touches the plane of the horizon but in a single point, that is in its north-eastern angle. Perhaps the time may come when, by strict attention to such observations, on a greater scale, we may hope to discover the laws which regulate the formation and directions of these vast ranges which hitherto have baffled the skill of our greatest geologists.

Lamellated Shist (Slate.)

This is very common, but, in general, the matter is soft. There is a pretty good quarry between Bond's-glen and Gossaden, past which a new road is now made. I have seen indications of this in Altahack-burn, in the Upper Burn-tolloght, in Learmount, and in many other places. The dip of the main quarry, of good slate, is frequently north-east; but the fossilist is liable to be puzzled, if he examines the inclination of the transversal divisions of the facets or tablets. Thus, a spectator, viewing the laminæ, into which the great couches are subdivided, without having knowledge or opportunity to consider the couches distinctly by themselves, would suppose the ascent of the quarry was to be measured towards the north-east, which, in reality, is but the transversal or lamellar cristallization of the couch, dipping towards the south-west.

The great mountain Sawel, is composed of several varieties of shistus. I traced not less than nine hundred perpendicular feet on its western ascent. Towards the summit, it is surmounted by shapeless masses of silicious shistus, with blocks of quartz, heaped without order, leaving interstices, which have the appearance of being caused by some former commotions; yet, I think, these cavities are occasioned by the destruction of the softer parts of the rock.

It would be an endless and an useless task to particularise every variety which may be found, belonging to this widely extended formation; its changes reach from the hardest silicious shistus bordering on the character of quartz or rock-crystal to that of the softest shistose mica, and fuller's earth or soapstone. As in most other parts of the earth, so also in this country, the great masses of our primitive mountains abound in that variety called mica-shistus. Nevertheless, this variety seems to be an interior stratification, which passes through several intermediates, both to the eastward (as in the mountains of Mourne), and to the westward (as in the ranges of the Rosses), till it has arrived at a granitic structure; these intermediate rocks are chiefly gneis and porphyry.

The chart will point out the great extent of this county, which is occupied by rocks of the shistose character, reference being made at same time, to the mineralogical characters inserted on the mountains and explained in the index.

Gneis.

This fossil, which seems to be intermediate between the shistose and the granitic structure, is peculiarly involved with the rocks of the former description. In the quarries, near the Fahan river, where it is abundant, its colour is that of blue grey; its stratification generally the same as that of the shist, that is to say, the mean couch dipping to the north-east. In the Glenrandle river, it is found of a waving and foliated structure, sometimes coloured, like red granite, and so much resembling this substance, in its grain and