THE HARDENING OF ROCKS 



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 crystal- 

 line. 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 pro- 

 duced 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 dis- 

 tinguished 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 solidi- 

 fied 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 



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