Imagens das páginas
PDF

HOW TO MAKE A GEOLOGICAL MAP.
BY HORACE B. WOODVVARD, F.G.S.,

OF THE GEOLOGICAL SURVEY or ENGLAND AND Warns.
[PLATE XCIII.]

[ocr errors]

AMONG the numerous efforts to popularise Geology, we are

not aware that much has been done to explain the mystery of geological maps. We say mystery, because by many they are looked upon as a sort of scientific myth; they are rarely understood by any but the geologist proper, and certainly are but little known and appreciated by our agricultural and “ practical” population. Of course to make the best use of our geological maps a considerable knowledge of the science is necessary; and if this were the case, we should not hear so often of fruitless efforts to find coal. The notions that black shales are an indication of coal, and that “where God has sent iron-ore, He has also sent coal to smelt it,” have not died out, anymore than has that relic of superstition, the Divining-rod, which in the West of England has still a few credulous practisers. But the meaning of geological maps may be understood without any very deep knowledge of geology. It is frequently remarked that such a map “looks very pretty with all those colours, but how do you find it out? ” To say a few words about the construction of geological maps, and what they mean, may therefore be of some interest; and in doing so, we may at the same time draw attention to a few of the practical bearings of geology.

In our own country we are well provided with geological maps, from the small atlas edition to our Government Survey Map, which, however, is not yet completed; and if it were, the map of England and Wales would measure about 36 ft. in length by 25 in breadth! It was about two hundred years ago (1673) when the first suggestion for a geological map of England was made by Dr. Martin Lister. Geology was then little known except as a mineralogical science, and fossils were either “ freaks of nature,” or the results of some “ plastic virtue ” in the earth. It is therefore interesting to consider

[graphic][ocr errors][ocr errors][graphic][graphic][graphic][graphic][graphic][graphic][graphic][graphic][graphic][ocr errors][graphic][graphic][graphic][graphic][graphic][graphic][graphic][graphic][ocr errors][ocr errors][ocr errors][graphic][graphic][graphic][graphic][graphic][graphic][graphic][graphic][graphic][graphic][graphic][graphic][graphic][graphic][graphic][graphic][graphic][graphic][graphic][graphic][graphic][graphic][graphic][graphic][graphic][graphic][graphic][graphic][graphic][graphic][graphic][graphic][graphic][graphic][graphic][graphic][graphic][graphic][graphic][graphic][graphic][graphic][graphic][graphic][graphic][graphic][graphic][graphic][graphic][graphic][graphic][graphic][graphic][graphic][graphic][graphic][graphic][graphic][graphic][graphic][graphic][graphic][graphic][graphic][graphic][graphic][graphic][graphic][graphic][graphic][graphic][graphic][graphic][graphic][graphic][graphic][graphic][graphic][graphic][graphic][graphic][graphic][graphic][graphic][graphic][graphic][graphic][graphic][graphic][graphic][graphic][graphic][graphic][graphic][graphic][graphic][graphic][graphic][graphic][graphic][graphic][graphic][graphic][graphic][graphic][graphic][graphic][graphic][graphic][graphic][graphic][graphic][graphic][graphic][ocr errors]

that at- this early period the practical value of a map, showing the areas occupied by each rock at the surface, was surmised. The first really practical map of the geology of England and Wales was that of William Smith, which was completed in 1815, after a labour of more than twenty years. This now forms the basis of the Greenough Geological Map, published by the Geological Society of London. The geological survey of the United Kingdom, which was commenced about the year 1832 by Sir Henry De la Beche, formed a new era in the history of geological maps. Of small maps of England, Scotland, and Ireland, we have those of Phillips, Murchison, Ramsay, Griflith, Jukes, and Geikie, but we need say no more about these. They pretend to no great minuteness or accuracy of detail, and are merely intended to give a general idea of the rocks exposed at the surface, so that practically speaking they will admit of but little improvement. Our Geological Survey Map, however, is on the scale of one inch to the mile; a few sheets are published on the scale of six inches to a mile, and here we find that detail which it would be impossible to show on a smaller map, and which it is often hard enough to show on the one-inch scale. It is these which are intended to serve as practical guides to the miner and the quarryman, the architect and the engineer, and the agriculturalist. It is to be feared, however, that these do not avail themselves of the maps to any very large extent ; certainly very few of our agricultural population ever heard of a geological map; and to judge from their general reception of the geological surveyor who is found wandering about in a mysterious way over their lands, they acknowledge but little respect for the science. “ What he doin’ of ?” a small farmer would say. “ Oh, making a map of ' the different rocks and soils,” one would reply. “Rocks and soils! I thought you was after the rabbits. You can’t come trespassing over these fields like that. Who be ye doin’ it for?” “ It’s a Government survey; I am at liberty to go anywhere,” would be the usual reply. “ Goverment survey, indeed,” says the farmer ; “ Goverment be always a doin’ summut queer.” While on this subject we may mention that returning one evening from field-work, we sat resting a few moments on a gate and laid our map-book on the adjoining post. Being absorbed in reflections not geological—for it is a relief to think of other things after a hard day’s work in the field, where all one’s attention is required—we arose and walked home to our humble village apartments, leaving our maps behind us. We soon discovered the loss, as we disencumbered ourselves of hammer, compass and clinometer, and at once hurried .back to the scene of our recent reverie. N 0 maps were there; it grew dark, and we returned home sad and dis

I

comfited. There was nothing for it but- to advertise; so we wrote out several large bills describing the maps, and offering a reward for their return, which we placed in the village shops.

. Two days passed in suspense; then, on the evening of the third, a farmer came in with them. He had found the map-case on the gate-post, and had put it into his pocket and taken it home; but here he did not stop ; he gave it to his little child to amuse itself with! Happily the maps were all right, unharmed, and we were thankful; though a sort of thrill ran through us as we thought of our maps, with three months’ work upon them, as a plaything in the unscrupulous hands of a child! So much for agricultural appreciation. Let us now turn ourtattention more seriously to the subject of geological maps.

It is well known how our British rocks are mostly of the “ stratified” kind—rocks deposited under water~such as slate and clay, limestone and marl, sandstone, sand, and conglomerate, intercalated one with another, and occurring at all horizons in the earth’s crust, generally harder or more compact the older they are. It is also well known how these rocks are arranged in a certain regular series, characterised by peculiar mineral characters, and 'more particularly by assemblages of

/ fossils which are more and more closely allied to the forms of life now in existence, the newer the rocks in which they are embedded. Thus we have the table of British strata, such as the old red sandstone, the lias, and the chalk; the oldest rocks known being the Laurentian, the newest including the alluvial deposits of our present streams. This order of superposition is never inverted, except by local and extremely rare disturbance, when the rocks may be folded or bent over so as to bring the older above the newer. We have also in England, though they occupy a comparatively small area, many “unstratified” rocks, the result of old igneous eruptions, and metamorphic rocks which in bygone times have lost their original stratified character through the agency of heat. These rocks include the toadstone of Derbyshire, and the granites of Cornwall. I

The arrangement of our stratified rocks has been aptly compared to layers of cloth of various colours and irregular shapes overlying and overlapping one another; some squeezed or rucked up with eVen layers deposited upon them, but yet arranged in a definite order comparable to that order of succession into which all stratified rocks may be determined. Each layer may be taken to represent a series of limestones and clays, or sandstones and conglomerates, or slates. Some layers-.— the lowest ones, perhaps—may be rucked up into mounds higher than all the rest, and yet they are clearly the oldest, because

« AnteriorContinuar »