382 O. K. LE ROY — GEOLOGY OF RIGAUD MOUNTAIN, CANADA 



To the south, in the apex of the triangle, the groundmass clianges, be- 

 coming a dull compact one. The Devils garden, in part at least, is 

 underlain by a variety transitional between the above rock and aquartz- 

 porphyr}^ which latter rock is developed east of the garden, continuing 

 in tliat direction until the S3'eniteis reached. The porphyry joints with 

 ease, and the fragments form an irregular talus pile along parts of the 

 northern slope. The contact of the porph3'ry and s\'enite was onl}^ seen 

 at two points in the extreme east; elsewhere it is concealed by drift, and 

 the approximate boundary line between the two rocks follows the deepest 

 part of the irregular valley which separates them. A small dike of ap- 

 lite cuts the S3^enite in the west. It is 2] inches wide and was traced for 

 62 feet in a northeast direction. This was the only dike noted in the 

 whole area. 



A small exposure of Paleozoic rock occurs in the bed of l-iver a la 

 Graisse, about a quarter of a mile north of the mountain. The beds are 

 transitional ones between the Potsdam and Calciferous formations; they 

 represent a thickness of about 40 feet, and dip south at an angle of 4 

 degrees. This exposure of stratified rocks is the only one in the vicinity, 

 and as they are not in contact with the igneous rocks nothing definite 

 can be deduced from them as to the age of the latter. 



In the southeast of the sheet and separated from Rigaud mountain by 

 the valle>' of the Racquette river there is an exposure of I^aurentian rocks, 

 consisting of an area of gneiss overlain in part b}' an amphibolite. This 

 outcrop of metamorphic rocks is due, according to Logan,* to an anti- 

 clinal axis, which divides longitudinally the great trough of Paleozoic 

 rocks lying between the Ottawa and Saint Lawrence rivers. Mont Cal- 

 vaire, a prominent outlier of the Laurentian. lies to the northeast of 

 Rigaud, on the opposite side of Ottawa river. The rocks on the slopes 

 facino; Rigaud are gneisses and amphibolites, much like the above, and 

 it is probable that these older rocks on both sides of the river are closely 

 related. 



The drift which surrounds Rigaud mountain and separates it from 

 the Paleozoic and Laurentian rocks has undergone a certain amount of 

 assorting l)y water, the mountain being flanked by a broad zone of gravel 

 and sand, which gradually pass into the sandy clays and clays of the 

 plain. Erratics are more common on the northern margin than else- 

 where, and consist chiefly of angular boulders of Potsdam sandstone. 

 Fragments of Laurentian rock, together with those of Upper Cambrian 

 age, are fouJid generally though S])aringl3" over the whole area. 



In mapping, therefore, the contact between the igneous rocks and the 

 drift was taken as the limiting line, except in such parts where it was 



* Geological Survey of Canada, report of progress, 185l-';j2, p. 8. 



