a) 
of much speculation, for no rocks of so recent an age are 
found in place anywhere in the western portion of the 
Province of Quebec. Since,however, it has been shown 
that in all probablity the matrix of the breccia was in a 
molten condition when it enclosed the fragments, and that 
the breccia as a whole has acted as an intrusive, the explan- 
ation is rendered comparatively simple. The breccia 
apparently represents the truncated pipe or outlet of a reser- 
voir of molten igneous material, which outlet may have 
reached the surface and even formed a subsidiary cone 
on Mount Royal, or else it may have been of the nature of 
a laccolitic mass, not opening on the surface. In either 
case the intrusion extended up into the Helderberg and 
Oriskany which must have overlain the Utica in this 
district. The intrusive mass stoped off blocks from these 
higher strata which either sank of their own accord to 
the level of Utica, or in the surgings of the magma, such as 
are to be seen in volcanoes to-day, were, at a time when 
the lava was sinking, carried to the lower level. 
The presence of these inclusions in the breccia proves | 
that the intrusion must have taken place subsequent to 
the deposition of the formations represented by the included 
fragments. The breccia is therefore of post-Oriskany age. 
Similar occurrences in which intrusive masses hold 
inclusions of fossiliferous sedimentary rocks more recent 
than any now exposed in the surrounding region have been 
described by Kynaston and Hall,* as well as by Peach 
and Gunn and others.** 
The Utica shale and the breccia of St. Helen: s island 
are cut by a number of dykes and small sills representing 
several varieties of the consanguineous dyke rocks of the 
Mount Royal intrusions. Among these is the faulted dyke 
referred to above as cutting the mass of Lower Helderberg 
limestone included in the breccia. This rock is basaltic 
in appearance and possesses both a porphyritic and an 
amygdaloidal structure. The amygdules are filled with 
calcite and analcite, the former mineral usually occupying 
the centre of the cavity. The rock is composed of pheno- 
crysts of pyroxene and hornblende, embedded in a ground- 
mass consisting of the same ferro-magnesian constituents 
with much analcite. The rock is a fourchite. 
*Diamondiferous Deposits—Geological Survey of the Transvaal. Report for the 
year 1903, D. 44. 
**On a Remarkable Vent of Tertiary Age on the Island of Arran, enclosing 
Mesozoic Fossiliferous Rock. Q.J.G.S. 1901, p. 226. 
