ABSTRACTS OF TAPERS 133 



able that the sillimanite did not develop until the intrusion of the magma. 

 This is borne out by the fact that quartzite in other portions of the quad- 

 rangle was not observed to contain sillimanite. The sillimanite, which is a 

 simple aluminum silicate, probably developed as a result of contact nieta- 

 morphism, the alumina possibly having been furnished by the granite magma 

 and the silica by the quartzite. This would explain why sillimanite is more 

 abundant in the lenses than it is in the large adjacent ledge of quartzite, 

 where the effect of metamorphism must have been less. During the intrusion 

 the material of the lenses probably recrystallized, and some of the newly de- 

 veloped sillimanite became included in the quartz, while some was carried off 

 into the borders of the magma. At the same time the tiny grains of magnetite 

 may have been introduced into the quartz-schist from the magma. 



It is a puzzling fact that the Grenville quartzite alone, of all the old rocks, 

 was cut to pieces in such a remarkable manner by the granite magma, and 

 this in spite of the fact that there are many large and small areas of very 

 intimately associated Grenville strata and granite or pegmatitic granite in 

 the surrounding country. 



The only other case of inclusions of sillimanite-schist in granite observed 

 by the writer in northern New York at all comparable to that above described 

 is on a ridge 2y 2 miles northwest of Indian Lake village ; but there the lenses 

 are not nearly so systematically arranged in the granite, and the sillimanite 

 is associated with considerable feldspar and some garnet. 



PRELIMINARY NOTES ON SOME REGULARLY BANDED ARGILLITE8 WHICH 

 SUGGEST SEASONAL DEPOSITION 



BY EOBERT W. SAYLES 



(Abstract) 



An examination by the author, in January, 1920, of the microscopic sections 

 of shale and slate in the National Museum at Washington with the purpose 

 of finding evidence of seasonal deposition revealed such evidence in a number 

 of cases. In the summer following, Mr. Allyn C. Swinnerton, a graduate stu- 

 dent in the Geological Department at Harvard University, was sent by the 

 writer to collect specimens and make a field study of the various argillites 

 chosen for investigation. A short report will be made on the characters of 

 the slates and shales studied. The following argillites show seasonal charac- 

 ters : the Hiwassee slate, of lowest Cambrian or latest Proterozoic age, from 

 southeastern Tennessee; the Athens shale, of Ordovician age, found at John- 

 son City, Tennessee, and other localities in that region; the Rockmart slate, 

 of Lower Ordovician age, found at Rockmart, Georgia ; a Lower Ordovician 

 or Upper Cambrian slate found at Melrose, New York ; a slate found at Schag- 

 ticoke, New York, and believed to be of the same age as the Melrose slate; 

 and a slate from West Pawlet, Vermont, of undetermined age, but probably 

 Lower Ordovician or Cambrian. Other shales and slates in the National Mu- 

 seum show seasonal characters, but no field study of these with the seasonal 

 theory in mind has been made. 



Read from manuscript, with lantern-slide illustrations. 

 The session adjourned about 4.40 o'clock p. m. 



