Читать книгу The Science of Brickmaking онлайн

12 страница из 23

Apart from differences of the character just described, serious alterations sometimes take place on these brick-earths being traced higher up the valley, and indeed an excellent brickmaking material may become absolutely worthless in that respect, for the reasons about to be explained. The reader will agree that neither stones nor sediment can travel up a valley, and he will understand that no sediment can be found in the valley earths other than that derived from the destruction of rocks within the watershed of the river system, to which the valleys belong, or did belong, at the time the earths were formed. We desire to put the case in a very simple light, so as to be clearly comprehended. Let us contemplate Fig.4.

Fig.4.—Map shewing river basin, with geological formations depicted.

Here we have represented a river basin, the limits (watershed) of which are indicated by a sinuous dotted line. Three geological formations are found therein; in the upper reaches of the main river is a series of clays marked A; a large tract in the middle, B, is sandstone; and the lower part, C, is occupied by limestone. Seeing that nothing but clay crops out in the part A, it follows that the deposits of the river in that region must be principally of an argillaceous character, to the point a. On flowing over the sandstone B, the main stream, already charged with clay particles, will be mixed with sand; the proportion of sand increases as the first large tributary (b) to the east is encountered, and is considerably augmented as the still more important tributary (b) to the west enters it. The superficial deposits in the valleys of the area B will likewise be very sandy and perhaps gravelly at b b, but at c c¹ the sands and gravels will be mixed with much clay. On passing over into the area C, much carbonate of lime is added, though the larger proportion denuded from the rocks is taken away, chemically, in solution. Nevertheless, nodules of “race” (lime concretions), limestone pebbles, and perhaps chert and flint gravel will come upon the scene at about the point marked e. At d the deposits would principally consist of gravel and impure marls. To sum up, the clays at a would no doubt be too stiff of themselves to make good bricks; similarly the beds at b b would be nothing but sand, though these might be made, with a little judicious treatment, into a species of fire-brick; at c we should find alternating loams and clays suitable for turning out fair bricks; at c¹ the beds would be more variable in character and more locally developed; they would consist of thin beds of sand, clays, loams and gravels (principally sandstone fragments), which as a whole might be made serviceable, though difficult to deal with; nothing of much use to us would come from point d, nor bordering the tributary running over C; there would be too much lime present, though a trade might be started in basic bricks should there be any demand for them in the neighbourhood; this, however, would only pay under extremely favourable conditions. At e there may be a mixture of all the foregoing deposits, and providing the beds above were easily weathered and thick beds of loam were thus fairly well developed, good sites for brick-earth might be found. The point e might possess this advantage over the other sites mentioned, viz., that marls would no doubt be present, and thus no necessity should arise for grinding lime to be incorporated with the brick-earth; the only danger would be that lumps of limestone might be too numerous—especially if c were a hard limestone.