What is (a) The molten matter within the earth, the source of the material of lava flows, dikes of eruptive rocks, etc. (b) The glassy base of an eruptive rock called?

magma — (Geol.) (a) The molten matter within the earth, the source of the material of lava flows, dikes of eruptive rocks, etc. (b) The glassy base of an eruptive rock. (Webster's 1913.)

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