49 
and frequently occurs as wreaths or coronas about the pre- 
viously crystallized olivines and augites. 
Rocks allied to alnoites also occur at La Trappe and 
Seo Ein, 
ANNOTATED GUIDE. 
Starting from the main gate of McGill University 
the party will walk through the southern campus which is 
situated on one of the marine terraces cut into the slopes of 
Mount Royal at the time of the post-glacial submergence. 
This terrace is 152 feet (46-3 m) above sea level. It con- 
sists of the Saxicava Sand underlain by the Leda Clay. 
The latter holds many shells of marine invertebrates which 
still live in the cold waters on the coast of Labrador. 
Among these may be mentioned—Saxicava rugosa, Leda 
minuta, Leda arctica, Mytilus edulis, Macoma_ gren- 
~landica, Balanus crenulatus, Mya truncata, Lepralia 
quadricornuta and others. These stratified post-glacial 
deposits rest on glacial till which in its turn rests on 
a glaciated surface of the underlying Trenton lime- 
stone. This deposit of till and stratified drift is 50 feet 
(15 m.) thick at the gate of the campus, but the surface 
of the underlying limestone gradually rises toward the 
mountain and the drift at the northern end of the playing 
field is only 18 feet (5-5 m.) thick. 
Leaving the University grounds by the western gate 
and passing up McTavish street, the basset edges of the 
Trenton limestone will be seen on the cliff behind the Low 
Level Reservoir. This body of limestone, when the reser- 
voir was being excavated some 37 years ago, presented a 
striking appearance, being intersected by a swarm of dykes. 
No less than 30 of these were mapped by Dr. B. J. Harring- 
ton in an area measuring 200 yards (182 m.) in length by 
100 yards (91 m.) in breath and were found by him to 
belong to at least seven periods of intrusion. They em- 
braced bostonites, tinguaites, camptonites, monchiquites 
and probably other allied types, which, however, were all 
found to be considerably altered, so much so in some 
cases that it was impossible to ascertain with certainty 
their original character. Analyses of three of these dykes are 
given on page 46). It was upon the wall of a large boston- 
ite dyke cutting the limestone here, that the mineral 
dawsonite was discovered. This rare carbonate of alumina 
32224—4 
