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. Limestone pierced by basalt becomes marble. When sandstone is discovered in contact with ancient volcanic rock it is found to have lost its reddish colour, and to have become white, grey, green, or black. It separates into crystals; it becomes glassy and hard. All these instances are those of rocks which we can perceive to have been altered by coming into contact with great heat. But there is another kind of change of a very much more widespread character which can be perceived among the most ancient of those rocks which we know must have been first quietly laid down as sediments. It is sometimes spoken of as "general metamorphism."