_Stereo Copyright, Underwood & U._ _London and New York_
The Pinnacled Castle-like Peaks of the Ramshorn Mountains Of Wyoming
The successive strata of sandstone are clearly evident in the peaks.]
For the present, however, we may concern ourselves with the condition of the earth and its rocks in Proterozoic times, observing merely that the remains of animals which we find there are of an order (Crustacea) which shows that life had progressed a good deal from its earliest beginnings in the age when the rocks containing these crustacea were laid down. After these rocks had been deposited they were subjected to many influences of which we have only dim conceptions. In a previous chapter we have compared the layers of the undermost and harder rocks of the earth to the lines on a page of this book as they would appear if the pages were crumpled up into a ball. Sometimes the beds of solid rock have been so distorted that they look like waves of the sea; sometimes they have been completely overturned; hardly ever have they been suffered to lie down flat. More than that has happened to them. Their very nature has been changed. This was done partly by heat, partly by pressure, partly by shock.
Let us consider the heat first. When a mass of erupted molten rock forces its way through the earth's crust, it produces effects which are quite easily recognised on the rocks it penetrates. Limestone becomes hard and crystalline. Rocks with silica in them become glassy and like quartz or other hard rocks which are sometimes polished to make ornaments. Clayey strata become baked into hard brick-like rocks. The changes are not altogether due to the heat. The eruption of rocks is accompanied by steam at high pressure and with all sorts of acids in the steam, so that chemical changes are also produced. It has been supposed by Sir William Crookes that diamonds, which are crystals of carbon, were produced by carbon being melted at some enormous heat under great pressure. Given the requisite conditions of heat and pressure, the parts of rocks which by their chemical composition are susceptible to crystallisation will form into enormous crystals--not unlike the intrusive rocks themselves. They can, however, be readily distinguished from the shapes which the intrusive rocks (like basalt) assume. The basalt rocks which form, for example, the Giant's Causeway in Ireland were volcanic lavas. Sometimes the lavas were masses which had solidified underground and had been thrust up by pressure from below or have been exposed by the weathering of the rocks above. Sometimes they have been lava poured out on the surface. The black compact kinds most often are seen in forms like columns. If these veins of volcanic rock have been thrust up through a bed of coal, the coal is changed or "metamorphosed" where the basalt has pierced it. Sometimes it becomes hard coal like anthracite; sometimes it is changed into graphite--the black rock of which pencils are frequently made.