400 PROCEEDINGS OF WASHINGTON MEETING. 



probable explanation. The consideration of those relations will now be taken up 

 in some detail. 



The northwestern half of the area in question is underlain by the orthoclase, 

 gneisses, and amphibolitic rocks of the Fundamental gneiss, presenting the char- 

 acters described above. A smaller area of the same gneiss occurs at the south- 

 western corner of the map, in the townships of Lutterworth, Snowdon, and 

 Glamorgan, while in the southern and southeastern portions of the area there are 

 other occurrences, which, however, are found to possess a more normal granitic 

 character. 



The southeastern portion of the area is underlain by rocks of the so-called 

 Hastings series, a series consisting chiefly of thinly bedded limestones, dolomites, 

 etcetera, cut through by great intrusions of gabbro-diorite and granite. These lime- 

 stones and dolomites are usually fine grained and bluish or grayish in color, with 

 thin interstratified layers holding sheafy bundles of hornblende crystals and, as 

 compared with the limestones of the Grenville series, are comparatively unaltered. 

 They form beyond all doubt a true sedimentary series, and in the southeastern 

 corner of the area are associated with conglomerates or breccias of undoubtedly 

 clastic origin. 



Between the great area of Fundamental gneiss in the northwest and the Hastings 

 series in the southeast of the sheet there lies an irregular shaped belt of rocks, pre- 

 senting the characters of the typical Grenville series as above described, the lime- 

 stones being in all cases coarsely crystalline white marbles, although often more or 

 less impure. The strike of the foliation of the Grenville series follows in a general 

 way the boundaries of the Fundamental gneiss. 



The relations of the Grenville series to the Fundamental gneiss are such as to 

 suggest that in the former we have a sedimentary series which has sagged slowly 

 down into and been invaded by intrusions of the igneous rocks of the latter series 

 when those were in a semi-molten or plastic condition. The contact of the Funda- 

 mental gneiss and the Grenville series would appear, therefore, to be in most 

 cases at least a contact of intrusion. 



The question of the relation of the Grenville series to the Hastings series in the 

 southeastern part of the area then presents itself. Although repeated traverses 

 have been made from one series into the other, no sharp line of division has as 

 yet been found. Toward the southeast the limestones of the Grenville series in 

 many places, while still highly crystalline, seem to be less highly altered, and 

 finally, as the Hastings series is approached, they present in places the bluish 

 color of the limestones of the latter series, so that it is often impossible to deter- 

 mine to which series they should be referred. The limestones of both series also 

 have the very numerous, little, interstratified, impure or gneissic bands so fre- 

 quently referred to in descriptions of the limestones of the Grenville series, making 

 the resemblance still more complete. In fact, although the true relations of the 

 two series are obscured by the presence of numerous great intrusions of granitic 

 and basic pyroxenic rocks, and can only be determined with absolute certainty by 

 the completion of the mapping, the investigations so far indicate that in the region 

 in question the Hastings series is the source from which the limestones and sedi- 

 mentary gneissic rocks of the Grenville series have been derived. The Hastings 

 series would seem to represent the series in its original form, which when invaded, 

 disintegrated, fretted away, and intensely metamorphosed by and mixed up with 



