394 



THE CIVIL ENGINEER AND ARCHlTECrS JOURNAL. 



[Dec. 



alse Tery important mirtures, known as loams and marls — the former being 

 admixtures of clav and sand, ami tlie latter of clay and limestone. With re- 

 gard to the next point, all soils were derived from the subsoil, and the mode 

 of derivation was a matter to wliich he drew particular attention. They 

 were derived by the chemical decomposition anrl mechanical disintegraUon 

 of rocks ; for the soil was connected witli and derived from the rock always, 

 either directly or indirectly. [The lecturer here referred to several dia- 

 grams, on which were depicted the soils, as they appeared in the course of 

 derivation from the fundamental rock.] In these cases the derivation was 

 direct; but, in others, when the material was brought from a great distance, 

 it was indirect. It was not, for instance, difficult to divine the way in which 

 the weather acted on the surface, or to understand how fragments of rock 

 misht be removed great distances, and deposited many miles from their 

 original beds. Every year vast quantities of material were deposited in the 

 Atlantic, brought liy icebergs from the Polar regions, and the materials for 

 future soils were thus obtained indirectly ; what they bad most to do with 

 ■was, however, obtained directly, as almost all soils in England, except gravel, 

 were obtained directly. Soils varied in thickness, from a few inches to 100 

 feet, er 200 feet ; but, ordinarily, were from a few inches to 3, 4, or 5 feet 

 ir» depth. The subsoil also varied a good deal in depth, which depended 

 very much upon local circumstances. Tlie lecturer then referred to the dia- 

 gram of a road section near I'enrbyn. The foundation was slate-rock, and 

 over that was a kind of rubble, into which the rock w.-is decomposed. Rub- 

 ble was a general term used to describe any rough disintegrated mixture of 

 rock, or broken fragments, with sand ; and from the rubble was derived the 

 subsoil, and from that the soil on the surface. In another section, of which 

 a diagram was exhibited, the slate at its usual depth lay in the ordinary 

 direction ; but, near the surface, it was " bent over." On this the lecturer 

 remarked, that it was impossible ever to determine the true dip of beds close 

 to the surface, as it was a common thing to finii it altered at the surface — 

 broken, in fact, as if by sonie mechanical force, and often inclined at a con- 

 sideraljle angle to the underlying b'-d. This often helped in the formation 

 of soils ; for the broken portions became mixed with sand and silt, and formed 

 rubble, from whence the soil was directly derived. In the diagram alluded 

 to, the next bed to the rubble was a loamy clay, which contained about 50 

 per cent, of sand. This, however, was not silica, and though it put on the 

 shape of clay, neither was it pure alumina ; but a silicate of alumina, mixed 

 ■with sand. Clay generally contained a good quantity of free sand, and when 

 mixed with about 50 per cent, of that substance, became loamy c'ay. In 

 that state it was better fitted for agricultural purposes, though it still re- 

 quired more carbonised matter to make it into soil. In the diagram, there 

 ■was about 2 feet of this loamy clay, which was called the subsoil, and above 

 that the true soil, which, in this case, was of a loamy nature, and contained 

 a rather large quantity of sand. Here, then, was the soil and subsoil directly 

 derived from the rock. The lecturer also further illustrated this point by 

 other diagrams, in which granite was the base from which the rubble was 

 formed. 



Soils varied much in value according to their different depths, and the 

 textures of their materials. M' hen the depth -was small they were liable to 

 Ibe carried away, or to be soon exhausted by the growth of vegetables on it ; 

 and then those particular ingredients, on which the vegetables subsisted, 

 ■were required to be replaced, or a further decomposition of soil, at a more 

 rapid rate than ordinary, became necessary. The application of other sub- 

 ■tances from a distance was thus sometimes necessary ; but, for this, some 

 chemical, as well as geological, knowledge was indispensable, or more harm 

 than good might be done. The texture of soils differed very much. Some 

 were exi-eedingly dense and heavy, and would not be easily washed away or 

 displaced ; but, though permanent, they were often very difficult to be 

 managed, particularly when they were so dense as scarcely to allow the roots 

 of plants to penetrate. Others, again, were so imperfectly made up, and so 

 large and coarse, as almost to preclude the use of the ordinary instruments 

 of tillage. Some soils contained a great deal of clay, and were so tenacious, 

 as scarcely to allow the plough, or even the spade, to act upon them. These 

 ■were exceedingly unmanageehle, for though it might be thought that a large 

 admixture of sand would les'^en this adhesiveness, it was generally found 

 that the sand, after a time, formed hard masses, and was apt to collect the 

 clay into lumps, instead of making it more loose. Some soils possessed a 

 large absorbent power; while others would allow water to pass through them 

 very readily. In soils of the latter kind valuable manures were soon washed 

 through them, without producing mnch effect. In these cases, a remedy 

 might be found in the practical application of geological knowledge, as, for 

 instance, liming the land, by putting on unburnt limestone in small lumps, 

 instead of slacked lime, and trusting to the slow decomposition of the lime, 

 stone by exposure to the wcatlier. There were many other circumstances of 

 a similar nature, such as the capillary powers of Ibe soil, or its aptitude to 

 crack and form great yawning chasms in times of drought ; the relations of 

 the soil with regard to beat, as its soon becoming hot like sand, or remain, 

 ing cool, or transmitting heat slowly, like clay. These points depended 

 almost as much on the substance that was below it, as on the texture of the 

 soil itself. 



The lecturer then proceeded to describe the soils derived from the various 

 geological formations in different parts of England. The districts, which 

 exhibited chiefly the igneous rocks, were the western parts of Cornwall, some 

 portions of Wales, and the greater part of Scotland, in all of which existed 

 some geological conditions, to be considered in reference to agriculture. In 

 composition these rocks were chiefly granitic, or, as it was called, porphyri. 



tic ; and they were made up of the crystal imbedded in a kind of paste, and 

 generally of crystals of felspar and mica, in a base of quarts. These rocks 

 existed in very different conditions, dependant principally upon the pre- 

 valence of the different ingredients, and their different decomposability. 

 First, they bad the quartz, often in compact masses, and so bard, that it was 

 exceedingly difficult to break. Where this mineral was in large masses, and 

 not much modified, it presented a most unfavourable condition for agricul- 

 ture, and indeed was almost hopeless, as it was next to impossible to get it 

 disintegrated by any natural exposure. When mixed with felspar, however, 

 the case was different, for that mineral contained substances of the greatest 

 importance in the composition of plants. When the felspar in granite de- 

 composed readily on exposure, it often formed a very valuable soil. Much 

 of the best soil in Cornwall was in this condition, particulariy that on the 

 lower hills, which, being most exposed to the operation of the weather, con- 

 tained the greatest quantity of granite in a decomposed state. They would 

 always find that the most fertile granite contained a good deal of decompos- 

 ing felspar. On the condition of the mica also might depend miic'i of the 

 disintcgrability of the granite. 



The next rock, he would consider, was gneiss, which contained the mite- 

 rials of granite in a mechanical condition. 1 hese would also form a fertile 

 soil ; but they might safely conclude, if it were hard and compact, that a 

 good soil wouhl not be likely to be formed. In the Highlands of Scotland 

 there was a vast quantity of gneiss and mica slate, and there the country 

 was uniformly barren on the hills, though there were spots which had been 

 made productive. The whole district, however, might be described as bar- 

 ren, aft'uriling support to little vegetable produce besides heather. 



.\nother class of igneous rocks were those which were forced up from be- 

 neath others, and were called intrusive rocks. Of this kind were the basalts 

 in Ireland, and the enormous masses of India, where there were 200,000 

 square miles covered with scarcely anything else. These rocks were nothing 

 more than lava, or melted rock, poured out upon the surface; they were 

 readily decomposable, and among the most useful and important ingredients 

 of soil they were not the least valuable. From these rocks were obtained 

 rich and fertile soils, as was exemplified by the districts in which Indian 

 cotton was grown. These rocks, which in England were often called trap- 

 pean, and which were probably poured out millions of years ago, were capa- 

 ble of being mixed with the soil in their neighbourhood. They contained 

 many of the materials most required by vegetables. Clay-slate was what 

 was called a metamorphic rock, and was, when in its simple and most charac- 

 teristic form, too little mixed with sand, and contained too few of the mate- 

 rials required by plants to be a valuable substance for soils. It contained, 

 however, a little iron, and sometimes a little soda, and other like ingredients, 

 but not ill such a state as would readily mix with carbon, and the gaseous 

 products necessary for plants. 



Beside- these, there were the oldest rocks, or Silurian rocks, which were 

 formed in Wales, twisted and contorted in every possible way. The lower 

 portions of these rocks were eminently siliceous, with a very small quantity 

 of alumina, carbon, and limestone, and a little potash and soda. Other parts 

 of the Silurian rocks had a great admixture of shales, with nodules of lime- 

 stone and large lenticular masses, distinctly traceable to local causes. These 

 rocks could he made fertile by careful admixtures. 



The Silurian rocks were succeeded by two other formations. The first of 

 these was the oM red sandstone, which was found in Scotland, and in cor- 

 responding beils in Herefordshire. This, when unmixed, was a very barren 

 rock, and oftentimes formed hills perfectly naked. Assisted, however, with 

 the calcareous lumps called corn stones, it formed sometimes a rich com 

 land. It was also well adapted for the growth of fruit trees ; and produced 

 the apples and pears from which the famous cider and perry of Hereford- 

 shire was made. Devonshire, which was also noted for eider, possessed the 

 same kind of soil, thougli of a different geological period. 



The Devonian rocks, containing a large quantity of silicious matter and 

 schists, which prevailed in Cornwall and the north-eastern parts of Devon- 

 shire, were also capable of being mixed with other substances, and rendered 

 fertile. These corresponded in age with the old red sandstone, but differed 

 in mechanical condition. 



Next to these succeeded the carboniferous rocks, of which there were 

 three distinct divisions. First, the carboniferous limestone, generally com- 

 pact in form, very hard, very little weather-worn, not easily disintegrated, 

 and not easily decomposed if not disintegrated, covered mostly with a very 

 thin soil, and well adapted for the purpose of sheep, and other animals, 

 which preferred a close line grass. Rich soils were seldom obtained by ad- 

 mixture with rocks of this kind. Next, there was the millstone grit, not so 

 coarse a conglomerate as the old red sandstone, and commonly better adapted 

 by mechanical condition for agricultnral purposes ; but not often forming a 

 rich soil, though covering a wide extent of country. .\nd, lastly, there were 

 the coal measures, of which the vegetable produce beneath the surface was 

 of far greater importance than the agricultural excellence of the soil. The 

 third class of carboniferons rocks were met with in the northern and central 

 counties, and in South Whales. 



The next in succession were the permian rocks, consisting of the mignCr 

 sian limestone, which generally coasted the coal measures. This substance 

 could never be safely used as a lime manure — caustic magnesia being always 

 mischievous. The soil derived from these rocks was not remarkable for its 

 richness. 



Then came the new red sandstone, which was necessarily a poor soil, coB- 

 sisting of unmixed loose sand, but which was often associated with a COB- 



