1870.] MACFARLANE — OX CRYSTALLINE ROCKS. 161 



brownisli-green transparent glass. Under a protecting cover of 

 dry sand, the solidified slag was found to contain crystalline 

 quadriitic prisms in considerable numbers, and between them lay 

 spherical concretions, consisting of regular radiating fibres, ex- 

 tending from the middle point in every direction. In the last 

 experiment the slag was exposed to slow cooling in a basin lined 

 with a warm mixture of charcoal powder and clay. When 

 broken, after cooling, it did not exhibit a trace of vitreous 

 substance nor any quadratic prisms, but a fine radiated texture 

 had spread itself equally throughout the whole mass. The ex- 

 periments of Sir James Hall have often been mentioned in con- 

 nection with this subject. Nearly seventy years ago he applied 

 experiment, for the first time, to the elucidation of geological 

 phenomena. It occurred to him to melt a small piece of basalt, 

 and the result was a dark vitreous substance. But on fusing a 

 much larger quantity, and allowing it to cool slowly, he obtained 

 a crystalline mass. Since that time geologists gradually became 

 accustomed to look upon the original rocks of a glossy appearance, 

 which occur in nature, as the products of rapid, and those of a 

 granular texture as the products of slow, cooling. Nor are there 

 wanting instances to show that other physical causes have in- 

 fluenced the structure of such artificial silicates as slaos. At the 

 Eglinton iron-works in Scotland, and those of Bethlehem, Penn- 

 sylvania, the writer observed that there is frequently developed 

 in the slags, as they flow from the furnace, streaked bands of 

 diff"erent colors, not at all unlike those developed in many slate 

 rocks. Then again, when the workmen, at the establishment 

 first named, tap off the iron and cool the small amount of scorise 

 which follows after it with a plentiful supply of water, the slag 

 froths up and solidifies to a porous cellular substance, the exact 

 parallel of which is to be found in the pumice stone of volcanoes. 

 In observing the slags of copper furnaces, nothing is more com- 

 mon than to see those which are allowed to flow over damp 

 ground rise up into porous scoria, while those which run over 

 wet portions of the smelting-house floor, boil up into loose pieces, 

 or throw themselves about in the form of little volcanic bombs 

 and lapilli. Similar phenomena are observed in the lava streams 

 of active and extinct volcanoes. Those of Alta Vista, in Teneriffe, 

 consist, on the surfoce, of glittering, transparent bottle-glass-like 

 obsidian, which, towards the interior, changes into a less glittering 

 pitchgtone-Uke mass, which is so filled with crystals as to resemble 



