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



intrusive*. In southeastern New York the very ancient Fordham gneiss 

 was of such variable character that earlier observers scarcely knew 

 whether it was igneous or sedimentary. The extended and careful 

 studies of my colleague, Charles P. Berkey, have shown it to be both, or 

 rather an ancient sediment soaked and rearranged into a mixed rock by 

 contributions from the near-by granites. In the neighboring parts of 

 New Jersey, J. E. Wolff had outlined the same history for the Pochuck 

 gneiss, 22a the probable equivalent, as we now believe, of the Fordham. 

 C. X. Fenner traced from the Pompton granite the same influence on 

 neighboring foliated rocks. 22b 



Recently, in a posthumous paper from our late fellow-member, Joseph 

 BarrelL 12 a paper to wdiich I have made reference above, the influence of 

 intrusive masses has been demonstrated in southern New England, in 

 localities where hitherto the strata have been regarded as regionally meta- 

 morphosed and as brought to their present state without the necessary 

 aid of igneous rocks. In discussing the mineralogical results, the author 

 utilizes the later results of physical chemistry. 



The invaluable excursions and discussions made possible by the Inter- 

 national Geological Congress in Stockholm in 1910 gave visitors from 

 this side of the ocean the privilege of seeing similar phenomena in Swe- 

 den We heard at first hand the older views from Tornebohm, and the 

 later interpretations from Hogbohm and Holmquist, with comments from 

 Sederholm, of Finland. With the remarkable work of the Geological 

 Survey of Finland, under Doctor Sederholm, Precambrian workers the 

 world over are now familiar and from it have gained much help. 



As the net result of all these observations and interpretations and from 

 the work of others whom I can not mention in detail, we are now well 

 assured that uprising igneous magmas are often charged with mineral- 

 izers which have been later freed in the process of crystallization and 

 have carried great quantities of dissolved matter into surrounding older 

 ^all-rocks, usually of sedimentary origin. The mineralizers have so pro- 

 foundly affected the wall-rocks that first one set of observers have thought 

 them igneous, and then another set sedimentary. The modern student 

 now recognizes them as both, and is sometimes unable to draw a sharp 

 line as to just where original sedimentary matter ends and contributions 

 from the intrusive begin. At points remote from the intrusive we find 

 augen or minute lenses of pegmatitic matter. Nearer the apparent source 

 the injections become larger and may afford the so-called "segregated 

 veins" of the older geologists. When such phenomena now confront us 

 we promptly look around for the intrusive mass of granite or related rock 

 which has caused the lit-par-lit injections. Where the intrusive was 



