as illustrative of Geology. 77 



above alluded to, which, as was first shewn in the Specimen Cry- 

 stallograpMee Metallurgicce, is produced not only by processes 

 employed in the manufacture of malleable iron and steel, but 

 by those also of copper-foundries. Among the blast-furnace 

 slags, too, are sometimes found such as are wholly crystalline, 

 partly foliated, partly radiated. Of this description is the 

 Kieselschmclz slags having the stoechiometrical relations of 

 pyroxene, and bearing an extraordinary resemblance to many 

 kinds of diopside in its external characters ; and likewise a kind 

 of slag which consists chiefly of a bisilicate of lime, and con- 

 sequently resembles the Wollastonite in its composition. 



The slags most rarely found, are those which are analogous 

 to the rocks composed of various crystalline minerals, in which 

 the ingredients of the fused entire mass have been separated ac- 

 cording to the fixed relative proportions on its becoming solid, 

 and by which the whole has become a union of different deter- 

 minate combinations. A formation of this kind is sometimes 

 presented by blast-furnace slags, in which crystals of kieselsch- 

 melz lie imbedded, in a foliated-radiated mass of pyroxene 

 slags, so that the whole bears some resemblance to the ne- 

 pheline dolerite, as described by Von Leonhard. In the 

 crystalline slags cavities are frequently found, in which cry- 

 stals are more or less perfectly formed, wherein the most 

 perfect analogy is displayed with drusy cavities in granite 

 and other mountain rocks composed of silicates. Now, if we 

 compare in general the phenomena observed on the slags com- 

 posed of silicates with analogous massive rocks, it must appear 

 remarkable, that among the plutonic mountain rocks, the cry- 

 stalline structure preponderates ; for we find the granular varie- 

 ties by far the most abundant, the porphyri tic and compact being- 

 much less frequent, and the glassy scarcely found at all ; where- 

 as in the volcanic masses the perfect crystalline structure is of 

 rare occurrence ; the porphyritic as well as the compact appear 

 on the whole more generally, and even the glassy kind is not 

 unfrequently met witli. To this must be added another differ- 

 ence, that a greater uniformity of internal structure belongs to 

 the plutonic than to the volcanic rocks. Here again, there- 

 fore, there is displayed a much greater analogy between the 

 products of our melting furnaces and more recent products of 

 the great subterraneous fire, than exists between the former 



