THE CHILDHOOD OF THE EARTH 



themselves at the surface as coast-lines of oceans and in 

 mountain ranges, after the sandstones and shales and 

 limestones had disappeared. 



The memorials of volcanic action remain, we had al- 

 most said permanently, among the decay of other rocks, 

 though, of course, even hard volcanic or igneous rocks 

 will be worn down in time. In many cases volcanoes 

 themselves are left, though they may have been for 

 ages extinct. In some volcanic regions where no great 

 central cones have existed the vast floods of lava that 

 were poured forth extend to-day as vast black plains 

 of naked rock, mottled with shifting sandhills, or as un- 

 dulating tablelands carved by running water into valleys 

 and ravines, between which the successive sheets of lava 

 are exposed in terraced hills. Beyond the limits over 

 which the lava sheets have spread there are often great 

 veins or parapets or sunken walls of lava to which are 

 given the names of igneous or volcanic " dykes."" Dykes 

 vary from less than a foot to one hundred feet in breadth, 

 and often run in nearly straight courses sometimes for 

 many miles. They consist usually of very hard rock 

 like basalt, " andesite," or " diabase."" 



They were fissures in the earth's surface, and, after 

 the manner we have described, the molten rock welled 

 forth through these fissures, and spread out sheet after 

 sheet, till like a rising lake it has not only overflowed 

 the lower grounds but even buried all the minor hills. 



Lava eruptions of this kind have taken place in recent 

 years in Iceland. On a small scale they can be seen to 

 take place in the island of Hawaii, where the outflows of 



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