84 
[20]. That pressure also has played some part in this 
metamorphism cannot be doubted, but the major part is 
undoubtedly due to contact action. The same applies 
to the metamorphism displayed by the limestones in the 
area to be visited, But here we are unable to compare 
the metamorphosed limestone with the normal, the latter 
being hidden from view south of the Ottawa river by a 
thick covering of Palzozoic sediments. 
The Grenville series is intruded by a great suite of 
granite and syenite batholiths, with subordinate diorites 
and gabbros which are included under the name Ottawa 
gneiss [21]. As a general rule the granites are older than 
the diorites and gabbros. The gabbros show all variations 
of composition from anorthosite to pyroxenite. In the 
small areas to be visited, typical granites are not well 
instanced. The oldest intrusives have characters which 
closely ally them with the charnockites of southern India. 
Later intrusives usually have the characters of diorite or 
gabbro. All of these have a more or less strongly marked 
gneissic structure, and are cut by many pegmatite veins. 
A younger set of gabbros, in which gneissic structure is 
typically absent, has been responsible for the formation of 
the ore-bodies to be visited. These are sometimes cut by a 
younger set of pegmatite dykes, which often possess peculiar 
characteristics. 
The youngest of all the intrusives, in the areas to 
be visited, are diabase dykes, which reach a thickness of 
60 feet (18-3 m.) or more, and can often be traced across 
country for many miles. Eastward, beyond the special 
areas to be visited, are large areas of anorthosite, the 
youngest of all the Pre-Cambrian intrusives. 
Ordovician.—The earliest member of this group is 
the Potsdam sandstone. It rests unconformably upon 
the Pre-Cambrian, and is followed conformably and some- 
times overlapped by the Calciferous sandy dolomite, which 
in turn is followed by the other members of the Ordovician 
system. The Potsdam sandstone is often quartzitic, and 
lacks definite fossils by which it may be correlated with 
the Potsdam to the south, which, in Wisconsin and else- 
where, contains the Dicellocephalus fauna. Up to the 
present worm-burrows and brachiopods are the only 
fossils found in the Canadian Potsdam. It seems prob- 
able that it represents the continuation, during the earliest 
Ordovician time, of the submergence of the North 
