78 JOURNAL OF GEOLOGY— SUPPLEMENT 



reaction to a minimum, should considerably advance the period 

 of precipitation of KAlSijOs as orthoclase/ This may be the 

 principal factor controlUng the essexite line of descent consisting of 

 gabbro, essexite, augite syenite, and the alkaline types common to 

 all lines of descent. Thus the precipitation of considerable ortho- 

 clase takes place at a stage when the plagioclase is still strongly 

 calcic in the case of essexite, and the delaying of those reactions 

 characteristic of great "activity" of water is still well marked at 

 the augite syenite stage, when the feldspar is already entirely 

 alkalic whereas the dominant ferromagnesian mineral is still a 

 typically anhydrous pryoxene. Even in this series, however, the 

 water finally makes its presence felt as its concentration in the 

 residual liquid increases, with the resultant production of feldspath- 

 oid-bearing and of quartzose types. 



An apparent association of rocks of the essexite line of descent 

 with specially intensive tensional faulting (e.g., Christiania and 

 fuKanehaab) lends support to the foregoing conclusion that this 

 line of descent is controlled by low pressures resulting in lessened 

 "activity" of water. ^ The rocks of this series as a whole have been 

 termed alkaline rocks, and if alkaline varieties belonged only in this 

 line of descent, Harker would probably be correct in concluding 

 that alkaline rocks are formed as the result of a certam definite 

 kind of tectonic movement. But whatever line of descent is fol- 

 lowed, there comes a time under appropriate conditions when the 

 concentration of volatile constituents brings about the production 

 of such alkaline rocks as nepheHte syenite. 



The essexite, augite syenite line of descent probably approaches 

 the extreme of low "activity" of volatile constituents at the early 

 stages, and the quartz diorite line of descent probably approaches 

 the extreme of great "activity" of volatile constituents at these 

 stages. Between the two there is presumably every gradation. 



' The calculations given by Iddings in Igneous Rocks, I, 152, are significant in this 

 connection. There he shows that the same analysis may be considered to represent 

 either a rock characterized by the hydrous minerals, mica and hornblende or a rock with 

 anhydrous minerals, pyroxenes. In the latter case the amount of orthoclase is nearly 

 doubled. 



^ It may well be, however, that volatile substances are as abimdant in magmas 

 of this series as in any others but that water is relatively an unimportant fraction. 



