Soft Rock and Hard Rock
Geologists in the 19th century did not dig deep into the ground to try to find oil or minerals. Travelling long distances through Canadian wilderness, they made most of their observations by studying the rocks and fossils they found at the surface.
How does a geologist figure out what bedrock lies underneath the soil, especially when the ground is covered with plants? Sometimes a geologist is lucky and can observe the geological structure directly. In the Rocky Mountains geologists can do that. Most often, however, they can only see outcrops (exposed rock) of the rocks that lie below the surface. This is what the "Rock Stars" found when they were exploring the southern parts of eastern Canada.
Geologists have to use all the clues they can find. Younger layers of sedimentary rocks are usually found on top of older layers. This is true except in a few tricky cases where the earth's forces have turned things upside down over millions of years. Usually though, rocks are found in layers that can be traced from one area to another. You might find a certain rock in two different places and know that they are connected as one rock formation. Rocks sometimes contain fossils. If you find the same fossils in two different places, then you have found rocks of the same age.
William Logan and his crew were looking for coal. Coal is found in sedimentary rocks, which exist in southern Ontario and Quebec. No wonder the Geological Survey of Canada was hoping for rich coal deposits there. Logan was able to prove very quickly that there was no coal because all the rocks in that area were too old and lay well below the coal-bearing carboniferous formations. Knowing this saved lots of time and money in useless digging.
Harder to figure out were the metamorphic and igneous rocks that make up the Canadian Shield. These rocks do not contain oil or coal, but do often contain lots of minerals and metal ores. Logan and his assistants took a look at these rocks, but they were not studied closely until the late 1800s and early 1900s.