The only question is whether it is entirely solid throughout. Some authorities consider this to be the case. But others venture to think that while the great mass of the globe is solid, there may be a thin liquid layer lying somewhere below the surface. Sir William Thomson calculates that there must be a solid crust at least two thousand or twenty-five hundred miles thick (the diameter of the earth is about eight thousand miles) and that the mass of the earth "is on the whole more rigid certainly than a continuous solid globe of glass of the same diameter." One other question with regard to the earth's interior may be mentioned in conclusion. Astronomers have calculated the weight of our planet, and the result is curious; for it turns out to be _at least twice as heavy as the heaviest rocks that are found on or near the surface_. It is about five and a half times as heavy as a globe of water of the same size would be, whereas most rocks with which we are acquainted are about two and a half, or at most three times heavier than water. This fact seems to open out curious consequences; for instance, it is quite possible that metals (which are of course much heavier than water) may exist in the earth's interior in considerable quantities. The imagination at once conjures up vast quantities of gold and silver. What is the source of the gold and silver, and other metals found in mineral veins? This question cannot as yet be fully answered. Very small quantities of various metals have been detected in sea-water; and so some geologists look upon the sea as the source from which metals came. But it is possible that they were introduced from below,--perhaps by the action of steam and highly heated water during periods of volcanic activity,--and that their source is far down below in the depths of the earth. But perhaps we have already wandered too far into the regions of speculation. Such are some of the interesting problems suggested by the study of mountains, and they add no small charm to the science of geology. And as we leave the mountains behind us, refreshed by their bracing air, and strengthened for another season of toil and labour by a brief sojourn among their peaks and passes, we come away with a renewed sense of the almost unlimited power of the unhasting operations of Nature, and the wisdom and beneficence of the Great Architect of the Universe, who made and planned those snowcapped temples as symbols of His strength, who was working millions of years ago as He is working to-day, and to whom a thousand years are as one day. INDEX. Agents of transportation, 161. Ages of strata, how determined, 317-333. Air, composition of, 209. Alpine animals, 124. plants, 103, 114. Alps, the history of, 330. (See also Ruskin.) Ancients, the, their dread of the mountains, 3. Andes, the, elevation of, 189. Animals, behaviour of, before an avalanche or earthquake, 95. "Anticline," 237, 303, 327.