coarse sand to fine sand, silt, clay, and calcareous mud. But along coasts which have many inlets or bays this order is not so well preserved, and fine silts or muds are sometimes formed quite close in shore; similar deposits of very fine grain occur also in the estuaries of large rivers.

In dealing, therefore, with clays and shales we must be guided largely by their fossil contents, and by the nature of the other beds with which they are associated, in deciding whether they are shallow or deep-water deposits. Black carbonaceous clays, for instance, containing much organic matter and associated with sandstones, would lead us to suspect the neighbourhood of a swampy shore or the estuary of a large river.

Limestones, again, though more often formed in comparatively deep water at a distance from land, are sometimes accumulated quite close to a coast-line, like the coral limestones of the present day. A pure limestone may be taken as a proof of clear water, and if there is reason to suppose that it was formed at no great distance from land, it may be assumed that no large rivers issued from the coast in question.

Every case in which limestones occur must be dealt with on its own merits, and no hasty conclusion as to the distance of land should be formed. There is, in fact, no rock-name that includes so many varieties as limestone, and the conditions under which a limestone may be formed are as numerous as are the varieties. That many of the limestones which occur among British rocks are shallowwater deposits is generally admitted; no one would claim a deep-water origin for a current-bedded oolitic limestone, or for the Llandovery limestones, which succeed sandstones and conglomerates, or for the Liassic limestones of Glamorganshire. The Carboniferous limestones of Northumberland and Scotland can hardly have been deep-water beds, and perhaps none of the Carboniferous limestone, even in

its more massive facies, was formed very far from land, though the water was doubtless clear and deep in those places where it attains great thickness.

Deep-water Beds. It is now generally acknowledged that the great mass of the rocks which compose our modern continents are such as are now only formed within 200 or 300 miles of land, and very seldom include any deposits which resemble those now accumulating in the depths of the Atlantic and Pacific Oceans. There is only one formation in Britain which was undoubtedly accumulated in deep water at a great distance from land of a continental character, and that is the Chalk. When, therefore, we speak of deep-water beds, it must be remembered that comparatively deep water is meant, and not water of oceanic depth.

Massive compact limestones and calcareous clays or marls have generally been formed in water of considerable depth. Many bluish and greenish clays, shales and slates, especially such as do not contain many fossils, have doubtless been formed in deep water, and are comparable to the blue and green muds which are found over certain tracts of the sea bottom, between the shallow water and the deeper tracts of the ocean.

Variations in Thickness.-The evidence derivable from the thinning and thickening of deposits also merits some examination. If we consider the case of sediment which is being transported by the action of a current setting off a coast-line, there can be little doubt that the greatest amount of sediment would be thrown down at a certain distance from land, where the bottom began to shelve into deep water. Little could be deposited in the shallow water near shore, but would be carried to a greater or less distance in proportion to the depth of the water; and if the bottom shelved gradually the deposit might cover a considerable space, but as soon as fairly deep water was

reached most of the sediment would subside, and little would be left to travel farther. A deposit formed under these conditions would form a large lenticular mass which would thin out principally in two directions, viz., in the direction of open sea and in the direction of land. Subsidence would make no difference to this arrangement, but would only tend to increase the thickness of the deposit.

The deposit would also thin out laterally, away from the central axis of deposition, but a transverse section across the mass would exhibit a different appearance from a longitudinal section; in a longitudinal section the size and nature of the particles composing the deposit would be nearly the same throughout, while in a transverse section we should find a gradation from coarse sand to fine mud.

When, therefore, we are dealing with a deposit, or a group of beds, which is thinning out in a certain direction, no conclusion should be drawn without having regard to their lithological characters, and whether these change in that direction or not. Thus, if we start with a thick mass of shale or clay, the materials of which must have been derived from the land, and we find that this simply becomes thinner without change of character or replacement by other deposits, we can only infer that we are passing away from the source of supply, possibly toward what was deep water, but more probably in a lateral direction from the major axis of deposition. If, however, as it diminishes in thickness, it becomes decidedly more calcareous, assuming the character of a marl and including beds of limestone, we may assume that we are proceeding away from the contemporaneous land and toward what was then an area of clear and deep water. If, on the other hand, a thick argillaceous deposit is gradually replaced by beds of sandstone, and these by pebbly and conglomeratic beds, there can be no doubt that the position of the contemporaneous land

is indicated by the direction in which the coarser beds set in.

We may note here that the formation is likely to be thickest along the tract where shales and sandstones alternate with each other, and as sandstones are more rapidly accumulated than clays, the formation may thicken landwards by the intercalation of sandstones; so that if only this portion of a group of beds is preserved to us, the direction of the land will be indicated by the thickening of the sandstones and the thinning of the shales. Among the Paleozoic rocks this is sometimes the only kind of evidence we possess to guide us toward the position of the land areas.

The Evidence of Unconformities. The existence of a widespread unconformity in any district, accompanied by the absence of certain groups of rocks which occur in neighbouring districts, raises the presumption that the first district was a land tract during the period which these rocks represent. Unconformities are therefore very important guides in the restoration of ancient geographies, because we may regard the eroded surface as the actual relic of an ancient land. It does not, of course, present all the physical features of that ancient land, because the surface has always been greatly modified as its successive levels came within the erosive powers of the sea under which it sank. Still we can often tell whether it was originally a high and hilly land, or whether it was of no great elevation.

In drawing inferences, however, from the absence of certain formations above a surface of unconformity, some caution must be exercised. If an unconformity occurs between two systems which are in geological sequence, such as the Ordovician and the Silurian, or between two parts of the same system, we must conclude that the older set of rocks was elevated during the epoch which is repre

sented by the missing groups.

And again, if a whole

system of rocks be absent, we may reasonably suppose that the area in question was land throughout the whole of that particular period.

If, however, the gap is very great, and several systems of rocks are missing, we must not conclude that the area has been continually above water during the whole of the periods of time which are unrepresented. The area may have been submerged more than once in the interval, and may have received deposits belonging to more than one of the absent systems, but all remnants of these deposits may have been swept away during the erosion which accompanied and followed the last elevation. Thus the Trias rests in many places on Silurian or Devonian rocks, but it is probable that in most of these cases the older rocks had sunk beneath the Carboniferous sea, and had originally been covered with some portion at least of the Carboniferous system; the Dyassic or Permian period, which preceded the Carboniferous, was one of great disturbance and denudation, and large areas of Carboniferous strata were then broken up and removed, so that the absence of these strata at the localities in question was probably due to this erosion. Such an unconformity may then be evidence of land in Dyassic times, but affords no clue to the position of the land tracts in the Carboniferous period.

In cases of unconformity the rocks which rest directly upon the old land surface are generally conglomerates or pebbly sandstones, the materials of which have been derived from the rocks of which that land was composed. Such conglomerates often cover very large surfaces, but it is well to remember that such beds are not the invariable accompaniments of unconformity. Thus, where a tract of land has sunk slowly beneath the sea for a long period of time, so that the newer deposits have overlapped one