PETROGRAPHY OF GRENVILLE AREAS 393 



Though the comparison of the analyses is hardly a fair one, it never- 

 theless shows that the two rocks are closely identical in composition. 

 A proper analysis of I would no doubt demonstrate that satisfactorily. 

 From the ratio of the alkalies in I, the feldspar of the groundmass is 

 evidently a soda-orthoclase. 



Relation of the Rigaud and Grenville Areas 



The brief description of the Grenville syenite and porphyry has shown 

 that they are closely related petrographically to the similar rocks of 

 Rigaud. The relations of the two areas are farther considered by means 

 of the following cross-section, which is a northwest prolongation of the 

 line ^ ^ on the Rigaud map. 



Rigaud, it is seen, is thus separated from Grenville by a narrow band 

 of Paleozoic rocks, which are supposed to overlie the Laurentian uncon- 

 formably. It would seem, however, by no means improbable that the 



G/tENVfLLE d A RiCAtfO 



yv r yr.- ..u + +.♦-* +K A ... - •.-.■.v «- * 4. ♦ <- *E 



Miles 



d, Laurentian yb, Syenite; c, PoRPHyRY; d, Pal/eozoic~Hor. Scale i^ «' i^ ■'^ >^ 



Figure 2, — Section from Rigaud to Grenville. 



two areas might be continuous under the Paleozoic, and that the strati- 

 fied rocks directly overlie the syenite instead of the Laurentian. Con- 

 nected in such a manner, the eruptive mass would have a length of 20 

 miles, with a known maximum width of 9. This would give an area by 

 no means enormous when compared with other masses of similar occur- 

 rence in the Laurentian, such as the granite intrusion northwest of lake 

 Saint Peter, river Saint Lawrence, which is 30 miles long and 12 miles - 

 wide in the broadest part. A point which favors this connection is that 

 Rigaud. from its field relations with Laurentian outliers, is practically 

 on the border of that system, and it is only to the south and west of 

 Rigaud that the Paleozoic plain is freely and uninterruptedly developed. 

 If the above association be true, then Rigaud is of pre-Potsdam age, and, 

 as in the case of Grenville, the eruption took place after the processes 

 which caused the metamorphism of the Laurentian system had ceased 

 and before the deposition of the Potsdam sandstone. 



Summary and Conclusion 



Rigaud mountain is the most western of a line of hills of igneous 

 origin. It difi"ers from the others in that, while they are of post-Silurian 

 age, its age is doubtful, as the contact with the Paleozoic is concealed. 



L VI— Bull. Geol. Soc. Am., Vol. 12, 1900 



