Advantages of Agricultural Geology.

395 raised from beneath an overburthen of more than twenty feet. The superficial deposits which are excluded from geological maps cover extensive areas, to depths varying from less than one foot to several hundreds of feet; and yet it has been long the fashion, among those who undertake to teach geology in its application to agriculture, to tell the farmers that the nature of the soil being given on one part of a geological formation, it is known for the whole; and that it is possible, by the mere inspection of a geological map, to announce the course of husbandry which will be found to prevail on the several districts represented on it. The intelligent farmer knows, however, that this is either not true, or a very rude approximation to the truth: and that within very small areas, on the same farm, and even in the same field, many varieties of soil occur of very different values, without any corresponding variation in the mineral character of the rock on which they rest. Is it surprising, then, that in a Farmers' Club the question should lately have been mooted, whether a knowledge of geology is of any advantage to the farmer, and that it should have been decided in the negative?

And yet the geology of agriculture is of the utmost value both to the owners and managers, and also to the cultivators of the soil. It aims at the establishment of a definite classification and nomenclature of soils, as a substitute for the local jargon, which deforms the best treatises on local agriculture, written by practical men. It aims at imparting a knowledge of the laws of the distribution of different sorts of soils. It aspires to the development of such a knowledge of the depth and composition of soils and subsoils, as will lead to the solution of the vexed questions, of the proper depth and distance of drains, and how superfluous moisture may best be removed, or water obtained when deficient. It aspires also to a knowledge of the distribution and composition of mineral manures, which afford the means of improving poor soils, and to the establishment of a general and precise nomenclature for them, as well as for soils, so that men in different districts shall understand what each is talking of when they use the terms clay and marl. It should be able to determine the respective influence on the soil exercised respectively by the rock formations and the superficial deposits; to discriminate between soils composed exclusively of the debris of the rocks on which they rest, and those in which the materials of several formations have been blended by aqueous transport. It should be able to afford assistance in the search for the best and cheapest materials for building, draining, and road making, objects of no small importance to the farmer; teaching him under what circumstances they may be sought with success beneath his own farm where their existence has not been suspected, and under what circumstances they may be obtained, in these

railway days, from a distance, of better quality, and at a cheaper rate, than that at which they can be procured nearer home. Lastly, it points out the indirect influence of geological structure on the value of land, by the industrial employments, of a non-agricultural character, to which the presence of certain strata gives rise, and the numerous consumers of agricultural produce which they cause to congregate in certain localities.

The investigations of agricultural geology should embrace, therefore, two distinct classes of facts-the composition and distribution of the strata of our geological maps, and the distribution and composition of the superficial deposits which are as yet unmapped. In the application of geology to agriculture, attention has hitherto been confined almost exclusively to the former. Attempts, however, have recently been made to return into the right path. Professor Johnston intimated, some years ago, that` the time had arrived when agriculture required geological maps of her own-maps which should include the drifts as well as the rock formations. This hint was acted upon by the construction, in 1844, 1845, and 1846, of maps in which the variations of soil were laid down on the Ordnance sheets, of a large part of Norfolk, and a small part of Cardiganshire; and subsequently, by the publication of Proposals for a Private Geological Survey, specially directed to Agricultural objects. A new class of geological maps was proposed, in which the variations of soil and subsoil should be shewn, on the private maps of estates, with greater minuteness of detail than is attainable on the scale of any public maps, except the Tithe maps of England and Wales, and the Ordnance maps of Ireland, and of a few counties in the north of England and Scotland, which have been constructed on the scale of six inches to the mile.

The maps of the surface geology of Norfolk, and of part of Cardiganshire, were undertaken for the purpose of determining the extent to which the variations of soil are dependent on the strata of ordinary geological maps, and on the superficial deposits. It was found in each case, that, excluding the alluvial class, a great variety of soils-soils worth a rent of more than thirty shillings, and less than two shillings the acre,—were irregularly distributed over areas in which there were no variations of the mineral character of the strata; and in which, had they existed, ordinary geological maps would not have shewn them.

The first of these maps was constructed as the basis of a paper on the Geology of Norfolk in relation to its Agriculture, published in the Journal of the Royal Agricultural Society of England. Having attracted the notice of Sir Henry De la Beche, who so ably presides over the Government geological survey of Great Britain, that distinguished individual became anxious to

Maps of Surface Geology.

397

combine the mapping of the geology of the surface with that of the substrata carrying on at the public expense. This new branch of geological surveying was accordingly commenced in South Wales; but after a few hundreds of square miles had been mapped, yielding the same results which had been obtained in Norfolk, and shewing a great variety of soils dependent on contours, where the map of the strata exhibited nothing but the colour indicating lower Silurian slates, it was first suspended, and then finally abandoned, in consequence of the reluctance of the Government to supply the necessary funds.

Professor Johnston himself has subsequently constructed two maps of New Brunswick, the one geological, the other agricultural, in illustration of the agricultural capabilities of the province, published under the auspices of the local legislature. More recently, Mr. Mylne has brought out a contour map of the London District, on which the surface variations are, to a certain extent, denoted, together with the mineral characters of the different beds of clay, sand, and pebbles belonging to the eocene tertiary formation,

It must be obvious, that independently of the economic value of maps of the surface geology, they would be the means of collecting, recording, and generalizing a vast mass of facts, of the utmost importance, in the investigation of questions in theoretical geology, affecting the history of the period known by the vague denominations of pleistocene and post-tertiary. We would define this to be the period intervening between the close of the Norwich, or Mammalian Crag, and the disappearance of the great mammals, whose remains are so extensively entombed in the wide-spread ancient alluvia of the Thames, and of most other rivers of Europe and America. This epoch is highly interesting from its proximity to our own times, and because it involves the history of a terrestrial surface during a protracted period; whereas in the older strata we see little but marine deposits, with such scanty and obscure traces of the existence of neighbouring land, as leave a wide field open to conjecture respecting its condition.

The true method, then, of ascertaining the respective influence which the rock formations and the superficial deposits exercise on the character of the soil, is to map them; laying down the latter with as much minuteness of detail as possible, together with the mineral characters of the strata on which they rest, and which are grouped on ordinary geological maps under one colour, as representing a common assemblage of organic remains. The true method of investigating the nature of the operations by which the superficial deposits were formed, consists, likewise, in mapping them. Not only should the areas covered by drifts be laid down generally, but the varying depth

and composition of those deposits should be shewn. By no other method than by such a minute examination can we learn what portion of them was formed beneath the sea, and what on the surface of dry land; whether disconnected masses of them are separate drifts-be the meaning of "drifts" what it mayor fragmentary portions of a once continuous sheet; whether, after their desiccation and denudation, they were exposed to any subsequent aqueous operations; and if so, of what kind, whether marine or atmospheric-whether produced by forces of the same kind and intensity as those now in operation, or by forces of the same kind, but different in degree. By such a course of investigation we should advance from the known to the unknown. At present we make the passage by a bound. We examine carefully operations now in progress, we measure the effects of existing forces, and we labour to bring every part of the ancient strata into strict conformity with the results obtained; vaulting dexterously over the interval between the tertiary and the modern epoch, and over any anomalies which that interval presents. Two opposite errors have retarded our knowledge of this period, overweening confidence on the part of some, that we are thoroughly acquainted with it, and a lazy despondency on the part of others, which suggests that the study of it is beset with difficulties so insuperable that it is hopeless to grapple with them. The one class think the problem solved when they have ascertained, on organic evidence, that during the later portion of the tertiary epoch there was a migration southwards of a northern fauna. The other class consider that the deposits of this era have been so broken up, and subjected to so many changes, that it is impossible to unite them into a continuous series. They would treat them like a geological puzzle, with the pieces of which they are well enough satisfied to amuse themselves, but which they will not take the trouble of putting together. We believe that those who will not shrink from this labour will find them as capable of being fitted into one another as the disjointed. fragments of a dissected map. We speak from experience when we say, that it is impossible to lay down the detached portions of clay, sand, and gravel in any extensive district, without finding the process reduce to order what previously appeared a mass of confusion. It is equally impossible to perform the same operations in several widely distant districts, without discovering an identity of character-the thread of a common clue-pervading the whole, and without being convinced, that if the same mode of investigation were extended to the whole island, the most sceptical would be convinced that the separate drifts of some geologists are the results of a connected series of operations during an epoch of considerable duration.

Northern Superficial Deposits.

399

In order to form some estimate of the importance, both economical and theoretical, of the superficial deposits, let us briefly trace their distribution over the northern hemisphere in general, and over the British Islands in particular; and let it be remembered, that when in the course of this survey we use the term erratic tertiaries it is to be understood as synonymous with the old term diluvium, and with the more modern term northern drift of some geologists and the "drifts" of others. In the north of Europe, from the White Sea to the German Ocean, beds of clay, sand, and gravel, accompanied by large boulders, which can be traced from their parent rocks in Lapland and Scandinavia, are spread over an area more than two thousand miles long, with a breadth varying between four hundred and eight hundred miles. They cover the whole of Belgium, Holland, and the north of Prussia, and occupy large tracts in Poland and eastern Russia. In Germany they approach, but do not actually reach, the fiftieth parallel of latitude. The boundary line of these erratics in Russia, as laid down by Sir Roderick Murchison, is very irregular. Its most southern point is between the sources of the Don and the Dnieper, where it extends to about N. Lat. 50°. It then ranges in a NE. direction to about E. Long. 57°, N. Lat. 61°, and then NW. to about E. Long. 48°, N. Lat. 64°. If any part of the British islands have been exempt from the operations of the erratic block period they are the steep escarpments of some ranges of hills and the lofty summits of others-the greater portion of the counties of Devon and Cornwall, the extreme south of Ireland, and the district south of the Thames-though even in these there are deposits of superficial gravel extensively distributed. In Britain, marine shells, generally in fragments, have been found in so many localities as to leave no doubt that a large portion of these deposits must have accumulated beneath the sea; and the species are such as to refer the beds in which they occur to that portion of the tertiary era which, in the nomenclature of Lyell, is denominated pleistocene, ninety-five per cent. of the molluscs belonging to species now living. There is evidence furnished by the forest of Cromer and Happisburgh, rooted in the mammalian crag, and buried beneath the whole mass of the erratic tertiaries, that this crag of the pliocene era had been converted into dry land, and had continued in that state for some time before it was submerged beneath the waters of the erratic sea. There is also evidence that the surface was inhabited by the mammoth, (elephas primigenius,) two species of rhinoceros, and other large pachyderms, of species now extinct, associated with others which cannot be distinguished by their solid parts from species still living.

On the Continent of Europe marine remains have been found but sparingly in these deposits, and that chiefly in Russia and