President's A ddress. 



193 



metamorphism of their own, and that they were originally 

 truly igneous like the basalts and greenstones- Place, for 

 example, the carboniferous and old red strata of Edinburgh, 

 with their associated porphyries, basalts, and greenstones, at 

 great depths in the crust, and subject them to metamorphic 

 action, and what would be the likely result ? Evidently, that 

 while the sandstones and shales and limestones and ironstones 

 were being converted into crystalline schists and marbles 

 and haematites, the porphyries and greenstones would also 

 undergo metamorphism, and be changed into granites and 

 syenites. So in all probability it has been with the old 

 crystalline schists and their associated granites — both have 

 undergone a metamorphism, but the former is not less sedi- 

 mentary, nor the latter less eruptive or igneous, because of 

 this internal mineral re-arrangement. Let us then clearly 

 understand what we mean by the term metamorphic, and 

 take care how it is applied. It is a convenient term for 

 altered strata whose age we cannot determine by the aid of 

 fossil organisms. It is a sound designation when applied 

 to altered rocks, whether sedimentary or igneous ; but its 

 action, however intense, does not affect the fact that such 

 rocks were primarily of aqueous and igneous origin. Some 

 granitic masses maybe merely highly metamorphosed schists, 

 but that does not affect the circumstance that others are 

 truly igneous, though altered in their mineral texture by a 

 similar metamorphism. The truth is, every substance in 

 the earth's crust is continuously and incessantly undergoing 

 metamorphism — the latest eruption of lava, as well as the 

 latest deposition of sea-silt, or the vegetable layer which 

 last summer's growth contributed to the gradually increas- 

 ing peat-bog. Heat by contact, heat by transmission, con- 

 duction, or absorption, heat by permeation of hot water, 

 electric and magnetic currents, chemical action and reaction, 

 molecular re-arrangement under pressure, and the like, arc 

 all conducing to this mineral change ; but though we fail to 

 detect in every instance the producing causes, we need not 

 on that account, and for the sake of some cherished hypo- 

 thesis, confound the obvious results. 



Passage Beds. — Another point of our science which re- 



