OCCLUSION OF IGNEOUS ROCK '^99 



morphic mica schists of the region unconformably and is be- 

 lieved by the author to be a highly metamorphosed intrusive 

 dike of Lower Silurian age. The original augite or diallage has 

 been completely converted into fibrous hornblende, and the 

 influence of pressure is shown in the perfectly laminated char- 

 acter of the schist, in the close foldings produced, and in the 

 minute structure of the rock."^ 



A more satisfactory explanation of the structural character of 

 the basic intrusive rocks at Philadelphia has been given in a 

 recent letter from Dr. Florence Bascom, of Bryn Mawr, Penn- 

 sylvania : 



" (i) The pyroxenic intrusives, that is, the gabbro, meta- 

 pyroxenite, meta-peridodite (or serpentines), occur in large 

 intrusive bodies. These intrusive masses, in general, follow 

 the. trend of the schists and gneisses, but when mapped can be 

 clearly seen to cross the strike of the bedding of the schists 

 and gneisses and to intrude indifferently into the mica-gneiss 

 (Hudson age) and the Pre-Cambrian gneiss. This can be seen 

 from the map, but in the field there are no good contacts. 

 (2) Smaller intrusive basic masses occur which occasionally 

 show contacts. Such an occurrence of a basic dike, now 

 altered to a hornblende-schist, is to be found in the neighbor- 

 hood of Swarthmore, on the Chester sheet. It strikes nearly 

 parallel to Crum Creek and at an angle to the strike of the 

 mica-gneiss. (3) There are other occurrences of altered basic 

 dikes, now hornblende-schists, in which the hornblende-schist 

 conforms perfectly to the schistosity of the mica-gneiss. In 

 these occurrences the hornblende-schist possesses inconsider- 

 able width and I first interpreted them as altered sedimentary 

 material. Upon further investigation, their petrographic and 

 chemical constitution proved to be similar to that of perfectly 

 well authenticated dikes and led me to consider these occur- 

 rences also as intrusive." 



In regard to the last case mentioned, intercalation of horn- 

 blende-schist in the foliation of gneiss, another possible explana- 

 tion might be based on relationship of foliation to bedding, as 



' H C. Lewis, Nature, 1885, 560. 



