Wild. — Geology of the Bluff. 335 



The analyses of the porphyry, the norite, and an intermediate member 

 of the schist series are repeated here for comparison. 



Porphyry. Schist. Norite. 



Si0 2 .. •• •• 67-60 61-00 48-10 



Al 2 6 3 

 Fe 2 0, 

 FeO 

 CaO 

 MgO 

 K,0 

 Na 2 

 Loss on ignition 



Totals .. .. 99-88 100-33 99-85 



Specific gravity . . . . • . • • 2-68 



These considerations — namely, the held relations of the rocks and 

 their mineralogical and chemical compositions — lead to the conclusion 

 that the hornblende schists are derived from the porphyry by meta- 

 morphism induced by the intrusion of the norite. The porphyry has 

 become sheared by enormous pressure, so that it has become foliated, 

 and its original character is masked. 



The thermal metamorphism of igneous rocks has received compara- 

 tively little attention, and geological literature available to me presents 

 no comparisons with the area to which this paper refers, and gives no 

 description of the chemical changes that take place in similar circum- 

 stances. 



In the case under consideration the principal changes to be accounted 

 for are the destruction of the phenocrysts of feldspar in the porphyry, 

 the devitrification of the glassy portions of its groundmass, and the great 

 increase in the amount of the ferro-magnesian constituents. 



The first two points can be explained by the ordinary processes of 

 hydro-, thermo-, and dynamo-metamorphism, all of which would be 

 active at the time of the intrusion of the norite. The water would be 

 partly magmatic and partly meteoric. 



The third point, however, involves the supply of large quantities of 

 calcium, magnesium, and iron for the formation of the ferro-magnesian 

 minerals, for the supply in the original porphyry was by no means 

 sufficient, and, in any case, the chemical analyses show that a large 

 quantity has been introduced. 



The norite we may at once presume was the store from which the 

 supply of these elements was derived, for the norite magma is very rich 

 in them. 



Transference of Material from the Norite to the Porphyry . — Perco- 

 lating water is universally recognized as a most potent agent, especially 

 at such high temperatures as would obtain in the case of a plutonic 

 intrusion. The small percentage of water in schists — 1 per cent, by 

 weight or 2 per cent, by volume — is held to be sufficient to account for 

 all the recrystallization that has taken place in rocks that are completely 

 metamorphosed. The solubility of minerals increases greatly when they 

 are in a state of strain. All these facts are well attested. 



