1.Perhaps you have seen men digging in the earth，either preparing for the foundations①of a house，or making a deep ditch or a railway cutting②.If you watch them at work，you will see that the earth which they turn up is not all of the same colour.
2.On the top there may be grass growing，and when the men cut through the tough network of its roots，they find a loose black or dark-brown earth which is easy to dig.This is called the soil.
3.When they go a foot or two further down，they reach a kind of earth which is much lighter in colour and much harder to dig.This is called the subsoil.If they have to dig still deeper，they will①Foundation，ground-work；that upon which anything rests.
②Cutting，road cut out under the ground or through rock.
come after a while to the hard and solid rock.In some places the workmen may have to dig down only a few feet before they reach the rock，but in other places both the soil and the subsoil may bemany feet deep.The depth varies①very much in different spots.
4.At one time the soil was just like the subsoil，and before that it formed part of some hard and solid rock.How are the hard rocks broken up and changed into soil？Chiefly by meansLIMESTONE ROCK（石灰岩）of water.Rain-water dissolves②the rock called limestone，just as your warm tea dissolves the lumps of sugar which you put into it，though more slowly.The tops of limestone hills are often worn into the most curious shapes by the action of the rain-water.
5.The rock called sandstone is made up of a great many grains of sand.The beating of the rain on this rock washes the grains apart，and so the hard sandstone slowly changes into loose sand.
6.All rocks are full of tiny cracks and pores.After rain these are filled with water.In winter it often happens that the rain is followed by frost，which changes this water into ice.Now the ice takes up more room than the water，and as it freezes it widens the cracks and pores of the rocks，and thus breaks off some pieces.
①Varies，changes.
②Dissolves，brings to a liquid state；melts.
7.The waves of the sea beat on the cliffs all round the coast，break off large pieces of the rocks，and dash them about on the shore，until at last they are broken into the tiny particles①whichwe call “sand”and “mud.”The rivers，too，which run over the land wear away their rocky banks in the same way.
8.Do you know why the soil is so much darker in colour than the subsoil and the rock beneath it？It is because a good deal of matter from decayed plants is mixed with the soil.The roots of plants，the leaves which fall off the trees，and the greater part ofthe manure②which we add to the soil，are all made of vegetablematter；and there is sometimes so much of this in the ground that the soil is nearly black.
SUMMARY
The soil is a loose black or dark-brown earth，and is easy to dig.Below the soil is the subsoil，which is lighter in colour and harder to dig.Still deeper down is the solid rock.The depth of the soil and subsoil varies much in different places.The soil and the subsoil once formed part of the solid rock.Limestone is slowly dissolved by rain-water，and sandstone is worn away by the beating of the rain upon it.The pores of rocks also get filled with water，which freezes and splits off pieces of the rocks.The waves of the sea and the current of rivers also wear away the rocks，forming sand and mud.The dark colour of the soil is due to the decayed vegetable matter which is mixed with it.
【中文阅读】
1．也许你看到过这样的场景：人们为了准备房屋的地基，或者建造一条深沟或者铁路而挖土。观察他们的工作，就会发现，他们挖出来的泥土颜色不尽相同。
①Particles，very small parts.
②Manure，a substance mixed with land to make it bear better.
2．土地的最上层可能长有一些花草，当人们的挖掘工具穿过这些花草强韧的根部之后，一层松软的黑色或深棕色泥土便露了出来。这就是土壤，它们非常容易挖掘。
3．当人们继续向下挖掘一至两英尺之后就会触碰到一种颜色较浅但很难挖掘的泥土，它的名字叫做底土。如果还需要继续深挖，不久之后就能看到坚硬的固体石头①。有时候，工人们可能只需挖掘几英尺便能看到岩石层；不过另外一些时候，岩石层上方的土壤与底土可能达到很多英尺之厚。在不同的地方深度相差很大。
4．曾几何时，土壤与底土一样结实，再往前追溯的话，它还曾经是坚硬、结实的岩石。如此坚硬的石头为何会碎裂并且变成土壤呢？主要原因就是水。雨水能够溶解石灰岩，就像温热的茶水能够溶解糖块一样，只是前者的速度要缓慢很多。在雨水的作用下，石灰石山的顶部会变成各种奇异的形状。
5．砂岩主要由大量沙粒组成。当雨水打下来的时候，这种岩石中的沙粒便被冲刷下来。久而久之，坚硬的砂岩逐渐变成了松散的沙子。
6．任何岩石中都存在许多细微的裂缝与孔洞。雨天，这些地方便充满了雨水。如果是冬天，雨水之后便会降霜，而缝隙中的雨水也就变成了冰。冰比水占据的空间要大，当它凝结时就会使得岩石中的裂缝与孔洞变得更宽。于是，一些岩石碎块便会掉落下来。
7．海浪反复拍打着海岸周围的崖壁，巨大的岩石碎块随着海浪来到了海滩上。经过岁月的沉淀，它们变成了细小的微粒，这就是我们所说的“沙子”与“泥土”。河流也是这样，当它们流过时都会将周围的河畔冲刷成这样的微粒。
8．你知道吗，土壤的颜色为什么比底土及其下方的岩石深那么多？这是因为很多腐烂的植物与土壤混合在了一起。植物的根部、树上飘落的叶子以及我们在土壤中添加的肥料，这些都是土壤中的植物成分。有时候，土壤中的这一类物质过多，土壤也就几乎变成了黑色②。
译注
①这种石头在岩土工程学中叫做“基岩”（bedrock）。
②有机质含量5%-10%的土叫“有机质土”（organic soil），其中的代表是黑土（black soil），我们最熟悉的就是北大荒的黑土地。如果有机质含量继续上升，达到了10%-60%，而且还含有腐烂的植物，这种土就叫“泥炭质土”（peaty soil）。60%以上就直接叫“泥炭”（peat）了。
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