240 J. F. KEMP AFTER-EFFECTS OF IGNEOUS INTRUSION 



readied the general conclusion that, aside from water gas, carbon dioxide 

 is the most abundant; nitrogen follows next; then the sulphurous gases, 

 hydrogen sulphide and sulphur dioxide, the former several times as 

 abundant as the latter; next, hydrochloric acid, hydrogen, methane, and 

 a trace of ethane. Except the water gas, which I mentioned in a pre- 

 liminary way to prevent misunderstanding, the above are not condensable 

 at ordinary temperatures and pressures. Under the head of condensables 

 come sulphur, salt, salammoniac, and ferric chloride. Gypsum is asso- 

 ciated, but is doubtless a result of a surface reaction. A long series of 

 other elements and compounds have been here and there reported — 

 tellurium, arsenical compounds, boric acid, sulphates of several kinds, 

 and all the common economic metals except gold and silver. All these 

 will have significance when we come to speak of the last stages of after- 

 effects. 



Comparison of effusive Emissions with those of intrusive Eocks 



As compared with the products from intrusive rocks, the most impor- 

 tant one which is lacking is silicon or silica. Silica must fail to pass off 

 with escaping gases at the temperatures and pressures of the surface, but 

 it goes freely into solution at lower temperatures in alkaline or other 

 waters, as the abundant siliceous sinters around hot springs demonstrate. 

 Under deep-seated conditions, as a fluoride or related compound, silicon 

 may escape at high temperatures. With falling temperatures and the 

 existence of liquid water, silica unquestionably passes outward in great 

 quantity, as will be subsequently brought out in speaking of contact zones 

 and mineral veins. These actual determinations of the mineralizers were 

 of necessity made on the emissions of effusive rocks. The samples are 

 caught at the surface and as they escape into the atmosphere. The sub- 

 stances dissolved in the water obtained in this way by condensing disso- 

 ciated hydrogen and oxygen, or steam, are only a partial indication of 

 what takes place around a deep-seated intrusive, under the high tem- 

 peratures and pressures attending a cooling and crystallizing magma and 

 under the precipitating influence of the ofttimes active chemical reagents 

 in the surrounding wall-rocks. We must learn of these processes by the 

 careful interpretation of their results. Several of the most significant I 

 wish to pass in brief review, and first the impregnated ancient strata 

 which have been studied and in later years interpreted in almost all the 

 Precambrian areas of the world. 



