. Elements of geology : a text-book for colleges and for the general reader. Geology. Fig. 2. changed into soil, the soil is also slowly carried away by agencies to be hereafter considered; and these changes, taking place more rapidly in some places than in others, give rise to a great variety of forms, some of which are represented in the accompanying figure (Fig. 2). In the process of disintegra- l ^SLl c Fig. 3.—a, vegetal soil: b. mineral soil; c. harder portions of rock left in process of disintegra- tion; d, underlying Fig. 4. tion the original blocks lose their prismatic form, an
. Elements of geology : a text-book for colleges and for the general reader. Geology. Fig. 2. changed into soil, the soil is also slowly carried away by agencies to be hereafter considered; and these changes, taking place more rapidly in some places than in others, give rise to a great variety of forms, some of which are represented in the accompanying figure (Fig. 2). In the process of disintegra- l ^SLl c Fig. 3.—a, vegetal soil: b. mineral soil; c. harder portions of rock left in process of disintegra- tion; d, underlying Fig. 4. tion the original blocks lose their prismatic form, and become more or less rounded, and are then called boivlders of disintegration. These may lie on the surface (Fig. 2), or may be buried in the soil (Fig. 3). When of great size and very solid, so as to resist decomposition to a greater extent than the surrounding rocks, they often form huge rocking-stones (Fig. 4). These must not be confounded with true bowlders and rocking- stones which are brought from a distance, by agencies which we will discuss hereafter, and which are, therefore, entirely different from the subjacent or coun- try rock. General Explanation.—The process of rock-disintegration may be explained, in a general way, as follows: Almost all rocks are composed partly of insoluble materials, and partly of materials which are slowly dissolved by atmospheric water. In the process of time, therefore, these latter are dissolved out, and the rock crumbles into an insoluble dust, more or less saturated with water holding in solution the soluble ingredients. To illustrate : common hardened mortar may be regarded as artificial stone; it consists of carbonate of lime and sand; the car- bonate of lime is soluble in water containing carbonic acid (atmospheric water), while the sand is quite insoluble. If, therefore, such mortar be exposed to the air, it eventually crumblec into sand, moistened with water containing lime in solution. Again, to take a case which often occurs
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectgeology, bookyear1892
