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Through it are dispersed rounded and sub-angular fragments of stone, from 3 to 9 inches in diameter, in such numbers that it almost deserves the name of gravel. In thin slices of volcanic glass one may see under the microscope the beginnings of crystal growth in filaments and needles and feathery forms, which are the rudiments of the crystals of various minerals. The above-mentioned combing and splitting action takes place at this point as well as for a distance of, say, 24 inches to 30 inches below. Repeat this, till it be as clear, and smooth, and white, as you would have it.