21st.—Caroline and I took advantage of a walk with my uncle this morning, to remind him of his promise to teach us something of geology.
“Are you prepared,” said he, “to learn the general classification? Though uninteresting till you know more, it is the necessary foundation to any knowledge of that science.”
“Oh yes, we are anxious to learn it, or any thing that you will be so good as to teach us.”
“Very well,” said my uncle; “we will begin at once. In examining the surface of the earth, a person would at first imagine that the confused variety of mineral substances he saw, was the result of mere chance; but if in different places he should find the same substances constantly linked together—if, for instance, in traversing the different coal districts, he were to find sand, clay, chalk, freestone, coal, limestone, sandstone, slate, and granite, succeed each other with tolerable uniformity, he would soon perceive that there was something like system in their arrangement. And on further examination, he would discover that this general series may be subdivided into several lesser series or formations, in which, also, considerable regularity may be observed. The order, then, in which these series are classed by geologists, is what I am now going to explain to my little girls.
“The first or upper series comprehends the mixed beds of sand, gravel, pebbles, and clay, which are frequently found covering the great chalk formation.
“The second class includes several different series more or less connected with each other: the most important of them are—1st, the chalk formation; 2dly, a series of sands and clays beneath the chalk; 3dly, a series of calcareous freestones, such as Portland and Bath stone; and, 4thly, beds of red marl and sandstone, sometimes containing alabaster and rock salt.
“The third general class comprises beds of coal and the limestones and sandstones on which they repose.
“The fourth or argillaceous class of rocks is characterised by their disposition to split into thin laminæ; such, for example, as the common roofing slate.
“The fifth, and lowest, contains all the varieties of granite and gneiss.
“These five series, or orders, have been named by one of our best geological writers, superior, super-medial, medial, submedial, and inferior. But the most general relation under which all these minerals present themselves, is that from which they have been named primitive and secondary. The primitive comprehend the lowest series of rocks, which serve as the bases upon which the others rest. They never contain any traces of former animals or vegetables, and may be supposed to have constituted the materials of the earth’s original surface.