THE PRECAMBRIAN ROCKS OP THE CANToN QUADRANGLE 5 1 



several different ways. They may be metamorphic ferromagne- 

 sian limestones, metamorphosed gabbros or gabbro-diorites, altered 

 diabase, or a contact product due to the diffusion of the basic mol- 

 ecules from granite into limestone. In the face of such a manifold 

 origin, it is evident that only in those cases where the data are 

 clearly shown, and the criteria decisive, can there be any approach 

 to certainty in interpreting these ancient rocks. On the Canton 

 sheet such data are wanting to a very large degree, and only rarely 

 is a rock exposure met with which betrays its own history. The 

 question is nevertheless touched upon here because it is by no means 

 impossible, however improbable, that some of the hornblende schist 

 which is considered as post-Grenville throughout the area of the 

 Pierrepont sigmoid may be of sedimentary origin. This is also true 

 of many smaller masses which are elsewhere conformably asso- 

 ciated or interbedded with undoubted Grenville. On the other 

 hand, it is difficult to demonstrate that these same masses are not 

 igneous. 



The criteria of transition, such as Adams and Barlow were able 

 to use with such success (Adams and Barlow, 1910, pages 166-70) 

 in tracing back the " f eather-amphibohtes " step by step to unaltered 

 magnesian layers in the Hastings limestone, fail utterly here on 

 account of the widespread recrystallization of all sediments in the 

 northw^est Adirondacks. The possible criterion of parallel inter- 

 banding would be displayed also by igneous sheets or sills intruded 

 along bedding planes in hydroclastic strata and does not necessarily 

 indicate interlamination of sedimentary material. Vertical transi- 

 tion, either from the underlying, or to the overlying, strata is ab- 

 sent, as is also interbedding of thin layers of amphibolite with lime- 

 stone, such as was noted by Adams and Barlow in certain localities 

 on the Bancroft sheet (Adams and Barlow, 1910, pages 164-66). 

 There is thus a general lack of evidence which is directly suggestive 

 of a sedimentary origin for any of the hornblende schist in this 

 quadrangle. Further, not much help is to be expected from purely 

 chemical or petrographical studies, although the detection of py- 

 roxene in a rock by the latter method would certainly suggest an 

 ultimate igneous origin for the hornblende schist in question. The 

 structural phenomena shown in the field, with confirmatory labor- 

 atory tests where possible, seem therefore to be those upon which 

 the geologist is thrown for a last resort; where these fail or are 

 ambiguous, the problem seems to be hopeless. This difficulty, 

 which appears to be insurmountable till more satisfactory criteria ' 

 shsll become available, is present with a large proportion of those 

 rocks which have been designated hornblende schists on the map, 

 4 



