TOPOGRAPHY 381 



the west of the peak. The torrents of water from the melting ice would 

 partially or wholly round the fragments of rock, and the currents, being 

 of sufficient force and volume, would carry away all the finer material 

 and deposit it farther south. After the retreat of the ice-sheet, and 

 during the subsequent submergence, these boulder plains would to some 

 extent have been worked over b}'' wave action, and thus arranged in 

 successive parallel ridges or beaches as they are found at present. This 

 dual origin seems to be the most probable, and more fully accounts for 

 peculiarities of deposition which are characteristic of both deposits than 

 if the whole work were ascribed to wave action alone. 



Another point of interest, and worthy of mention, is a fissure spring 

 which occurs on the northern ridge about half a mile east of the Devils 

 garden. It is 500 feet above sealevel, and from its height and its being 

 unaffected by seasonal variations it would appear that the water supply 

 is derived from some higher land, in all probability the neighboring 

 Laurentian country. 



General Geology 



The greater part of Rigaud mountain is composed of hornblende- 

 syenite, which is pierced in the northwestern part by an intrusive mass 

 of ])orphyry. The syenite is a uniform, massive, coarse grained rock, the 

 exposed surfaces of which vary in color from pale red to grayish white. 

 The rock joints into rough rectangular blocks, which are very numerous 

 along the eastern margin. Red chert veins cutting the syenite are com- 

 mon in all parts of the area. They never exceed half an inch in width, and 

 follow irregular courses, from a few inches up to a hundred feet in length. 

 Along the northwestern margin a miarolitic structure is developed, evi- 

 dently quite similar in character to that found in the Baveno granite on 

 lake Maggiore. The cavities are of all sizes, from the most minute up 

 to those having a diameter of 2 or 3. inches. The}^ either contain well 

 formed crystals of feldspar and quartz — the former occurring as Baveno 

 twins — or crystals of quartz, to the total exclusion of the feldspar. This 

 structure seems to indicate that, while the syenite is of deep seated origin, 

 the then existing pressure was not so enormous as to prevent the forma- 

 tion of these cavities, which are more common in younger rocks of this 

 class, such as the granite of the Castle Mountain mining district, Mon- 

 tana, described by ^yeed and Pirsson. The porphyry occupies a roughly 

 triangular area, surrounded on two sides by the syenite. The most prom- 

 inent part of the ridge (figure 1, plate 33), which faces the Ottawa river, 

 forms the broad base. The rock shows considerable differentiation of 

 magma. West of the Devils garden it is a quartz-syenite porphyry, 

 which consists chiefly of feldspar phenocrysts in a felsitic groundmass. 



