As yet we have only been _talking_ about the crust of the earth; we shall now return and enter upon its actual examination. It will not be necessary for us personally to descend into the abysmous caverns that lie beneath our feet, nor, with hammer in hand, to go forth and explore the district of country in which we may happen to dwell: we may do all this by and by, when we know both _how_ and _what_ to observe. Meanwhile, with such teachers as Buckland, Sedgwick, Murchison, Pye Smith, Hugh Miller, De la Beche, Lyell, Owen and others, we may for some while to come be only tarry-at-home travellers; for in a true sense, in this department of knowledge, “other men have laboured, and we enter into their labours.” Let us now look at the crust of the earth, as it may be represented in two imaginary sections. Suppose we could make a vertical section of the earth’s crust, and cut straight down some eighty miles till we reached the central mass of incandescence that we believe lies beneath this crust, or Erdrinde (_earth-rind_), as the Germans call it, and then bring out this section to daylight, it would present something very much like the following appearance. [Illustration: DIAGRAM I. ] Here the granite A will be observed forming the supposed boundary between the superlying strata and the fire B below to which we have just referred, and thus will be seen the origin of all plutonic rocks. Here too will be seen how the granite is not confined to the lower levels, but rises, as mentioned in the first chapter, far above all the other strata, and forms some of the highest peaks on the face of the globe.[6] Here, too, will be seen how the granite is frequently traversed by veins of trap-dykes, those black-looking branches, which rise often above the whole mass of metamorphic and stratified rocks, often occasioning great difficulties in mining operations. Here, too, the student will see how, supposing the theory of a central globe of heat to be founded in fact, the volcanoes that are now active, C, form, as the volcanoes that are extinct, D, once did, the safety valves of this mighty mass of incandescence, B; and in the same way may be seen how certain strata may be above the granite, or above any other formation, though they do not overlie them, and how the lowest strata, being formed first, is said to be older than any superlying strata, notwithstanding any accidental arrangement produced by upheaval or depression. For, in “consequence of the great commotions which the crust of the globe has undergone, many points of its surface have been elevated to great heights in the form of mountains; and hence it is that fossils are sometimes found at the summit of the highest mountains, though the rocks containing them were originally formed at the bottom of the sea. But, even when folded or partly broken, their relative age may still be determined by an examination of the ends of their upturned strata, where they appear or