78 Professor Hausmariil on Metallurgical Phenomena 



and earlier productions of the latter, a circumstance which it 

 is easy to account for by observations made on the structure of 

 slags. The plutohic rocks are manifestly formed by a very 

 slow cooling; and since their masses are partly of very large 

 dimensions, the cooling might take on the whole a more uni- 

 form course, and consequently produce greater uniformity in 

 the internal structure. Towards their boundaries the plutonic 

 masses display frequently the influence of a more rapid cooling. 

 Hence granite and syenite on their boundaries have often a 

 porphyritic character ; and the large-grained granite has some- 

 times a coating which is fine grained, and passes into hornstone. 

 The volcanic rocks underwent, on the contrary, a more rapid 

 cooling, and that, indeed, in proportion to the recency of their 

 formation ; their masses, too, have usually a much smaller com- 

 pass. Hence, in regard to them, we often observe in inconsider- 

 able spaces a remarkable variety of internal structure ; hence, 

 for instance, in vein-like masses of dolerite the interior is often 

 crystalline, whereas the external part appears compact. 



An additional circumstance appears to have exerted special 

 influence on the predominant crystalline formation of the plu- 

 tonic masses, viz. the prevalence of silica. The formation of 

 several silicates, which are distinguished for their great tendency 

 to crystallize, as well as that of mica and felspar, was thereby 

 effected, and in case of an excess of silica, the entire mass 

 might pass into crystalline combinations, the free silica hav- 

 ing become separated in the form of quartz. With a diminu- 

 tion of the quantity of silica, came also into play, in the case of 

 the plutonic rocks, a diminution of their tendency to chemical in- 

 dividuality and crystallme structure, as is observed in the horn- 

 blendic and pyroxenic rocks. The same consequence undoubt- 

 edly ensued in the case of a very large superabundance of silica, 

 whereby the formation of eurite and hornstone porphyry was 

 chiefly occasioned. 



We shall now once more revert to the most usual kind of 

 slags, viz. the glassy, in order to attain a more accurate ex- 

 planation in regard to certain phenomena in volcanic masses. 

 The form and structure of these masses, as well as of the glassy 

 lava, are entirely accommodated to the laws of common attrac- 

 tion, and are consequently essentially different fi'om that which 

 appertains to crystallization. The chief form of the amorphous 



