Park. — Contact Hetamorphism. 17 



the contraction will tend to cause the two rocks to shrink 

 from each other, resulting in the formation of cavities along 

 the line of contact. 



Above a temperature of 365° C. and a pressure of 200 

 atmospheres, water and all more or less volatile compounds 

 will exist as gas. Aqueous vapours above the critical tempera- 

 ture and under great pressure will act as strongly upon the 

 cooling magma as upon the adjacent rocks. They will possess 

 a solvent power which will be greatest at the depth where the 

 highest temperature and pressure are reached. The pressure 

 will cause the heated steam and gaseous emanations carrying 

 the heavy metals to permeate the bedding-planes of the sedi- 

 mentaries, and fill all accessible cracks and fissures. In this 

 way bed-impregnation may be effected, and even ore-bodies 

 formed at points some distances from the genetic eruptive 

 magma. A decrease in the temperature and pressure will 

 cause the least soluble substances to be deposited ; and as 

 the temperature and pressure continue to diminish, the dis- 

 solved substances wdll be thrown out of solution in the in- 

 verse order of their solubility. It is manifest that the later 

 phases of the eruptive after-actions will represent in a modi- 

 fied form the waning effects of solfataric action. The deep- 

 seated conditions will also favour the action of metasomatic 

 processes in the zone of metamorphisrn, and veins will be 

 formed, some of which may rise to the surface. It is probable 

 that the circulation of the heated mineralised solutions in 

 the later phases will tend to effect a redistribution of the 

 ores and minerals deposited in the earlier stages. In some 

 cases the ascending waters and gases may reach the zone of 

 surface circulation and mix with the meteoric waters, which 

 will then reappear as hot springs, forming ore -bodies and 

 veins not directly in contact with the eruptive magma. 



Weed and some other writers have made an attempt to 

 ' subdivide contact-metamorphic deposits into groups depend- 

 ing mainly upon the mode of occurrence. But the form and 

 mode of distribution may be due to accidents of density or 

 porosity, composition and hydrous condition of the rocks 

 affected, rather than differences in genetic formation. More- 

 over, the mass of the magma, the weight of superincumbent 

 rocks, the amount of heat and subsequent contraction, and 

 phase of the after-action are all doubtless contributing factors 

 in connection with the form and distribution of the heavy 

 metals. Masses of ore occurring as contact deposits, fissure- 

 veins, and bed-impregnations in the zone of metamorphisrn 

 may all be traced to the same genetic causes. 



Professor L. de Launay, of Paris, supports the views of the 

 school of De Beaumont and Daubree in respect to the primary 

 influence of volatile mineralisers emanating from eruptive 



