would still more easily admit the application of the same reasoning, from the contiguity of their abrupted parts; but the detail would be tedious; those who wish to pursue the subject farther must come to the scene themselves.

Materials completely carried off.

A circumstance perhaps still more extraordinary, is the complete removal of all the materials that once filled up the intervals between the abrupted parts of these strata; I have stated in my 9th fact, that the materials that had formerly composed the projecting parts of our northern façades and precipices had totally disappeared.

The removed parts of the limestone stratum on the west side of our area have shared the same fate, for where the chain of mountains extending from Magilligan Rock to Bienbraddock, is interrupted by vallies at Stradreagh, Drumrommer, and Ballyness, it is obvious that the limestone stratum was once continuous to the high points where it shows itself on Keady, and the mountains on each side; its thickness too, wherever we can try it, is very great; yet this stratum, which in its entire state must have spread like a roof far above the present surface of these valleys (which are now sunk deep into the schistose substratum) has not left a particle of its debris behind, nor is a single lump of white limestone to be found until we come to the quarries, that is, to the edge of the solid, untouched stratum.

Conclusions.

The conclusions that unavoidably follow, from the consideration of these facts, are,

That the hills and mountains, in the district that I bave been describing, were not raised up or formed as they now stand, but that they are the undisturbed remains of strata that were left behind, when stupendous operations carried away the parts that were once contiguous to them.

That the inequalities of this surface were all produced by causes acting from above, and carrying off whatever they touched, without in the least disturbing what was left behind.

Additional Evidences. Basaltic Hummocks.*

The arguments on which I have founded my opinions have hitherto been all taken from the hollows in our surface, and the interruptions in our strata, both which the concomitant circumstances have led me to consider as so many excavations; but the lofty elevations, and the abrupt prominencies rising suddenly from our surface, when minutely examined, lead us irresistibly to the very same conclusion.

When you and I examined together the line of our northern façades, we studiously sought for the points where nature had made any change in her materials or their arrangement, hoping that at the junctious of these little systems, we should find some facts that would throw light on the subject; but we generally failed; want of perpendicularity, or other local circumstances, impeding us at the most interesting points.

On the present occasion she has adopted an opposite line of conduct, and in many of the steps she has taken, obtrudes upon us demonstration of what she has done.

Whoever has attended to the exertions of man, when employed in altering our present surface, either by levelling heights, or by making excavations, must have observed that it is the practice of the workmen to leave small, cylindrical portions standing, for the purpose of determining the height of the old surface, and thereby ascertaining the quantity of materials removed.

To these may be compared the stratified basaltic hummocks so profusely scattered over our area, and which seem to shew how high our quondam surface once reached.

The hummock of Dunmull, three miles south-east from Portrushi, is very beautiful, it stands on the top of a high ridge, and is a couspicuous object from all parts of the country; it is exactly circular, its flat surface contains an acre, it is completely surrounded by a perpendicular façade about twenty-five feet high, and formed by two strata, a columnar, and an irregular prismatic, resting upon it.

From this elevated station, where I had the pleasure of accom

* Navigators use the word hummock to express circular and elevated mounts, appearing at a distance; I adopt the term from them.

panying you, I shewed you at six or seven miles distance to the westward, among the Derry mountains, the still loftier hummocks of Altabrian and Sconce, hemispherical in form, composed of but one stratum each, while their swelling-out bases displayed accumulations of many more: and also near those the hummock of Fermayle, resembling Dunmull, but much larger, and also surrounded by a façade composed of two strata,

I shewed you also at twenty miles distance to the south east, the gigantic Slemish, one of our basaltic hummocks, magnified (as it were) into a lofty and insulated mountain, completely stratified from its base to its flat summit.

I shewed you likewise from the bottom of its ridge, the neat but diminutive hummock, called the Rock of Clogher, above Bushmills. As our time was precious, you took my word for its stratification being precisely similar to that of Dunmull.

There are many other basaltic hummocks scattered over the surface of our area, all of them either stratified or portions of strata; two of the most remarkable are the hill of Knock Longhran, near Maghera, and a tall hummock (whose name I forget) a mile eastward from Lisamore.

We meet still more frequently an imperfect style of hummock, a semi-circular façade ou one side, while on the other it slopes away gradually with the dip of the strata, as if the operation had been interrupted before it was carried quite round; the most remarkable of these are Ballystrone, in Derry, and Croaghmore, in Antrim, both visible from Dunmull.

Regular stratifications on the summits of hills and mountains, have been long a stumbling block to theorists; the historian of the French Academy, for the year 1716, obviously ascribing the superficial inequalities of the earth, (like many others) to causes acting from below, and perceiving how incompatible such assemblages of strata were to his theory, thinks it safer to doubt their existence, as they could not have been formed, he says "unless the masses of the mountains were elevated in the direction of an axis perpendicular to the horizon: ce que n'a pu être que très rare."

But as these stratified mounts are in our area by no means uncommon, they lay us under the necessity of suggesting another alternative similar to those we have already stated.

Were these isolated hummocks originally formed as they now

stand, (solitary and separate from each other) one by one; or, are they the last remaining portions of a vast consolidated mass, of which the intermediate and connecting strata have been carried off by causes with which we are unacquainted?

To be able satisfactorily to resolve this alternative, it becomes necessary to take a careful view of the contiguous countries, and to try whether their construction, and the arrangement of their strata, will throw any light upon the subject,

When we examine the assemblage of hummocks above Knockmult, that is, Sconce, Fermoyle, and Altabrian, we find their materials and stratification precisely similar to that of the country below them to the eastward, where the abruptions of the strata are displayed in long stony ridges;-to the south, the abruptions on the summit of Keady mountain discover the same similarity; and to the north-west the grand façade of Magilligan Rock, three miles distant, displays an accumulation of exactly the same sort of strata consolidated into au

enormous mass.

The hummock of Dunmull is formed of two very particular strata, a columnar, and an irregular prismatic; but I shewed you, a mile to the northward, at the façades and quarries of Islamore and Draigahuller, at the base of the hill, that the whole ridge, on the summit of which Dunmull is placed, was a consolidated mass, formed by alternate strata of the same description; and that the arrangement of the whole country below, and adjacent, was precisely the same with that of the hummock of Clogher, I proved to you at the curious opening of the strata at Bushmills Bridge, and in the façades at the Giant's Causeway.

After these proofs that so many (and I might proceed to the rest) of our detached hummocks are in their construction and materials, similar to, and connected with, the main consolidated masses of which our country is formed, I think it will scarcely be asserted that these hummocks were originally formed, solitary and separate as they now stand: but rather that they were once parts of that vast whole, and left behind in their present form, upon the removal of the contiguous portions of their strata, by some powerful agent, of whose operations and modes of acting, we have hitherto obtained little knowledge

The highest point on the façade of Cave Hill is called M'Art's Castle, and appears to be a solitary fragment of a stratum pre

cisely similar to those below it, and obviously once extended like them.

The irregularity of the summit of Fairhead, plainly shews that its gigantic columns once reached higher.

And in the façade of Magilligan, the highest of all, a few desultory patches of an upper stratum (no doubt once perfect and continuous) are to be traced along its summit.

Our mountains themselves seem to shew clearly that they were once higher; the top of Magilligan mountain is an extensive plain, over which a wandering stratum is interrupted and resumed at intervals for a great way.

At the highest part of Donald's Hill, over the valley of Glenuller, we find a hummock; also at the termination of the ridge, at its highest part over the valley of Mayola, similar hummocks.

[Phil. Trans. Vol. XCVIII. 1808.]

CHAP. V.

GEOGRAPHY, OR THE DOCTRINE OF THE GENERAL FACE OF THE EARTH,

IN

N the sense in which we have already stated it is our intention to use the term GEOLOGY, Geography holds the same relation to it as Geognosy; the former constituting that department of Geology which contemplates the actual surface of the Earth, as the latter constitutes that department which describes its apparent origin and chemical structure. We shall briefly contemplate the science under the two sections of its history and its principles.

SECT. I.-History of Geography.

The study of geography being of so much practical importance in life, must have commenced in the early ages of the word. It was regarded as a science by the Babylonians and Egyptians, from whom it passed to the Greeks, and from these to the Romans, the Arabians, and the western nations of Europe. Thales of Miletus, in the 6th