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



"the Jack in every alley" among the igneous rocks, has chlorine or 

 fluorine ; that the socialite group has chlorine and the noselite group the 

 sulphuric acid radicle; so that a little of the mineralizers may remain in 

 the composition of some characteristic but subordinate igneous rock- 

 making minerals. The great proportion of the igneous rock-makers are 

 practically devoid of them, certainly so far as notable percentages go. 

 We therefore conclude that the process of crystallization is a process of 

 eviction, and that with the advance of solidification nearly all the dis- 

 solved gases are gradually left uncombined. They are evicted first from 

 the cooling and consolidating exterior of a batholith or laccolith. As 

 crystallization extends inward and downward, they continue to be freed, 

 since we do not find them in the congealed mass, except as mentioned 

 above. They doubtless escape through crevices in the shell, very probably 

 along trunk channels, and pass into or through the wall-rocks, much 

 more abundantly at some places than at others. 



This conception appeals to me rather more strongly than the one re- 

 cently stated by our lamented fellow-member, Joseph Barrell, 12 in which 

 an advance guard of escaped or escaping mineralizers is believed to pre- 

 cede the upward course of the molten mass. Possibly no sharp line can 

 be drawn between the two views. In a matter so largely speculative, one 

 may hold opinions or beliefs in a plastic state ; but in the discussion of 

 the subject in the present paper the point of view will be held that the 

 mineralizers and their dissolved burdens are evicted and escape on con- 

 solidation of the parent magma. 



After-effects ix crystallized igneous Rocks 



REACTION RIMS AXD RELATED PHEXOMEXA—DEUTERKs 



Perhaps the very earliest of the "after-effects" may be observed in the 

 intrusive itself. The microscope has often revealed to us the reaction 

 rims which are developed in the gabbros and more basic rocks. Zones or 

 crowns or aureoles of minerals of intermediate composition are found 

 between feldspars on the one side and magnetite, pyroxene, or some 

 other basic component on the other. Garnet, hypersthene, and biotite 

 are very frequent in the rims, and a zone of untwinned feldspar, clear 

 and colorless, may be often seen. 13 



Again, in the diabases and gabbros we sometimes note brown biotite 

 along the contact of magnetite and bordering feldspar. J. J. Sederholm 

 described years ago peculiar micro-pegmatitic intergrowths of vermicular 

 quartz and plagioclase fringing larger feldspar nuclei in granites and 

 gneisses. He called the intergrowth myrmekite. 14 All the minerals of 



