ERUPTIVE EOCKS. b07 



Contact metamorphism. — There is a iiotablo absBiice of caustic phenomena 

 in this region, either on the inclosing or the included sedimentary rocks at 

 their contact with the intrusive masses, such as are generally supposed to 

 accompany the eruption of igneous rocks. 



In the case of such numerous and large bodies they might naturally 

 be expected to be exceptionally frequent and well marked, since the eruptive 

 masses must have retained great heat for an unusually long time on account 

 of the depth at wliich they were consolidated. Perhaps the absence of any 

 evidence of fusion in these rocks might be explained on this very ground, 

 that at that depth the pressure was so great that the fusion point was con- 

 siderably raised, and hence a temperature sufficient to hold in a molten 

 condition the mixed material already fused would be insufficient to melt 

 homogeneous and by themselves comparatively refractory rocks, like sand- 

 stones and dolomites. However this may be, nowhere was any evidence 

 of fusion observed in the sedimentary rocks, even in the case of very small 

 fragments entirely included in the eruptive rock. Even in the dikes of por- 

 phvrite cutting through the Archean, in which inclosed fragments of coun- 

 try rock, generally of small size, are particularly abundant, neither quartz, 

 granite, nor gneiss, of which these fragments generally consist, shows any 

 alteration at the contact, though the porphyrite material often fills small 

 cracks in them, showing that it was in a thoroughly fluid condition at the 

 time they were caught up. Such alteration as was found could more 

 lendily be ascribed to the combined action of heat and water than to heat 

 alone. 



On the other hand, the reflex action of the colder seuimentary rocks on 

 the eruptive mass is generally noticeable, and is such as is ordinarily found, 

 showing itself in a fine-grained or even compact structure for a few inches 

 or more from the contact, and in a somewhat different arrangement of the 

 mineral constituents of the rock. It is much more prominent in the narrow 

 dikes than in the large intrusive sheets, but occurs at both upper and lower 

 contacts of the latter, and may also be detected around the included frag- 

 ments. Even in these cases, however, there was no appearance of vitrifi- 

 cation 



