272 A. C. LANE — GEOLOGIC ACTIVITY OF THE EARTH's GASES. 



telluric phenocrysts are cori'odecl is a well known fact, and that acid 

 and basic quartz and olivine are alike corroded seems to point to some 

 such neutralization of the magma. The presence of the quartz in quartz- 

 basalt can readily be explained, and thus some of the so-called basic 

 secretions and volcanic bombs may be due merely to imperfect mixture. 

 Richthofen's law,^ that lavas of medium basicity first appear, would find 

 its basis in the obvious fact that the material first delivered would be of 

 a mixed character if the crack were suddenly opened. I would not, 

 however, deny the cooperation of liquation and splitting in j^roducing 

 varieties of magmas. 



Source and Diminution of acid Eruptives. — The next point, the shallow 

 origin of exclusively acid eruptions, has already been discussed in con- 

 nection with the correlation of basicity and heat ; but it is significant 

 that we have no olivine-porphyries corresponding to quartz -basalts. The 

 laccolitic form, so common in acid rocks, may also point to a less degree 

 of energy than the enormous outbursts of basalt. It does not seem as 

 if there were ever, certainly not in later times, so extensive overflows of 

 rhyolites as of basalts, and this would be a necessary consequence if 

 the acid is a superficial layer, with less gas to give oft" than the layers 

 below, and continually growing cooler and stiff'er. 



The effect and indications of higher temperature in basic rocks have 

 already been discussed. Is the frequent greater size of the vesicles in 

 basic lavas connected with relief from a greater pressure? 



Contact Zones. 



Intrusive Rocks. — So far we have treated mainly of volcanic discharge 

 of gases. Let us now look at their behavior in cases of intrusion. If 

 there is a crack or line of weakness not leading to the surface, the gases 

 will first escape into it, and then, if it is relatively cooler, decrease in 

 pressure, so that the magma from below will force in after them ; but it 

 will not be able to fill the space entirely unless the gases are able to 

 escape into the w^alls of the cavity. Thus there may be a gradual settling 

 of the rock into place, producing the " protoclastic," " Mortel " and 

 similar textures.f If the lava be quite viscous and the cavit}^ a na.rrow 

 fissure, it may leave a vacant place. This summer I saw such a place 

 near the head of McCargoes cove on Isle Royale. In this case a little 

 intruded dike was continued by a vein of comby quartz. 



Beside the forerunning gases we have those of the magma itself to 

 account for, which may be done in three ways. One way is to suppose 

 that they too pass into the wall rock. Then we have that characteristic 



* Geikie : Op. eit.. pp. 202, 263. 



tBrogger: Zuits. fur Kr. 1890, p. luo. Adams: Neues Jahrbuch, Beilage Band, viii, 1893, p. 459. 



