AGRICULTURAL GEOLOGY.

Dunstan Times, Issue 1214, 5 June 1885, Page 3

AGRICULTURAL GEOLOGY.

(Continued from Last Issue.) By W. Gum. The durability of * rook or its power to resist weathering depends upon its mineral character. Bocks which contain over 60 per cent, of silica in their composition withstand the disintegrating influence of the atmosphere for ages, and consequently have very thUpaoils upon them. In this class will bo'found the older crystalline rooks, sud'sbtne of the igneous rooks, as granites, trachytes, etc. And generally speaking, the more mineralised a rook is, the thinner will be the soil, and the scantier the vegetation upon it, and vice vertn. Still there are exceptions to this rule. On the other band rocks which contain less than 60 per cent of silica are more easily weathered and accumulate soil, sometimes to very great depths. A certain class of erimtio rocks termed “ pyroxenic,” in which are included dolerites and greenstones, some of whom closely resemble granite for texture and hardness, yet are found to weather very last in a damp atmosphere. In the Longwood range. Southland, I have, seen upwards of 20ft of a loamy clay lying upon greenstone rocks. The clay had aoannnilated solely by the decay of the rock hi situ, after the manner in which rust is formed upon a piece of iron if left undisturbed in a moist situa ti»n. Rocks possessing a slaty cleavage, contain innumerable joints, which are receptac’es for water, and under the action of frost are easily split and crumbled to pieces. To this class belong the clay s'ates and the mica schists of Otago and Southland. The decomposition of the slate rocks result in a deposit of clay having a large percentage alumina and silicia. with oxide of iron and a very little lime. It is at best rather discouraging to the agriculturalist requiring expensive drainage and the importation of sand lime and organic remains to maintain its fertility, being considerably deficient in the nece-sary mineral elements which support vegetation. The mica tchtsle contain lime, potash, soda, and the oxides of iron and manganese, which they yield abundantly to the soil Where depth of soil occurs upon these rocks the noil is almost permanent by fertile, containing the necessary elements tor the sup port of vegetation requiring only the volution of crope and organic manures. The soil found upon limestone rock is c mparatively poor, but the soil of the' valleys and plains of a limestone district through the admixture of foreign elements is generally fertile. The I arreness of a soil is generally due to the excess of some mineral element and to the absence of of O' S. The •older volcanic regions are generally fertile, with luxuriant vegetation, but the newer are very barren. The character of a soil may often be judged by the natural vegetation found upon it. In districts of soin altitude ** speargrass ” grows upon good light loamy Soil ; and flax upon wet sandy loams, while manuka is generally in cold wet clays. The tree fern luxuriates in a soil derived from eruptive rocks, often accompanied by the rata or “ iron-wood.” Instances of this nature are often noted by the careful observer. We may summarise this paper by adding a few remarks on

WHERE GOOD SOIL KAY SB GENERALLY FOUND. Plains that open oat to the sea usually liave a good depth of soil upon them And river plains have greater depth of soil, and are more fertile as they approach the const. ’That loyrdyiog ground by the sides of alow rivers called “flats ” or “ hot!oms," enriched by occasion I floods, is generally valued for grazing ground for cattle. On the tot s of the older terraces of the lake basins of Otago and Southland, there is •comm-mly found a good depth of f rtile soil, but the middle of the plains of ihe ol I lakes is often shingly ; Still theie is a good sprinkling of gravelly soil among tha shingle. - There is also good black vegetable mould on the tops of the Otago mountains, where it culminates in a peat bog, Down the long sloping spurs of these mountains there exists a good back loam, while along their bases is usually the best ground of the valley ot plain. Except on the more mineralised parts of the low ranges, a good productive soil can often he found in situations where the rocks are sticking out of the ground, giving tne idea of barrenness to the scene.

The same degree of fertility cannot however, be predicted for the ranges of tbe ■clay slate formation. Occasionally we have seen farmers settling down on the low ridges from these ranges, in situations that from the mineral character of the soil must prove very disappointing to them after a few years. The best soil in this formation is found along the edges of the plains. In some parts of Southland, on sune ot the spurs and flats of the clay lands, there are numbers of small water holes termed “ pot holes,’ thickly netted about. The holes are s' out 6in in diameter, and from one to two feet deep. To horsemen they are quite as perfect traps as the rabbit holes. Geologically the holes indicate a surface soil of a cold wet nature, with a substratum of clay impervious to water Some patches of good flat land are to be found in the little bays and shelterel nooks along the sea coasts. The soil in these situations is com manly formed by the admixture of eruptive rocks wi hj beach sand and lime from decaying shells; it is fertile except around saltwater lagoons. About the estuaries of rivers there are generally large tracts of fiat land, which enterprising settlers now and then settle upon. But such ground should be taken with a caution, both saltwater an-1 bad climate are sometimes in opposition to the prospects. Peat bog is in some situations ground not to be despised. It holds water to the top like a sponge, but when the fibre is broken by cultivation, and a few large drains cut through it, the water soon evacuates the cultivated stream, and it bi comes one ot the most p/*anently produntive soils tjiat is to be found. In the fern country ot Lincolnshire we have seen splendid crops raised upon such ground. Lime and bone dust are suitable manures for it. Although we have in this paper only treated on the character cf the soil as derived from the rooks under it, s ill we do not lose sight of the fact that there are many situations where our rule does not hold good, on account ot the improvement in tbe soil and the distribution of it by rivers and other agencies, Dis tricts have been enriched this way which otherwise would have remained barren. There are also other conditions which materially affaet the maturity of plant life, besides the fertility of the soil, such as the altitude of the district, the rainfall, and the drainage system, and not least, the amount of light which pours down from the sky upon the landscape. Agriculturalists would do well to study these natural influences which sooner or later must tell upon their prosperity.