Is there limestone under the Oregon Coast Range?
I'm not familiar with the Oregon Coast Range, but limestone is such a ubiquitous and widespread rock that your friend is almost certainly right. For strata to be limestone, they need to have been under the sea when they were laid down. The fact that some are found today near the tops of mountains only means that plate movements have uplifted them since they were formed. They might easily have become overlaid with igneous rock, volcanic tuffs, sandstone etc,but if you go down deep enough the probability is that you'll find limestone somewhere. You usually don't have to go down very far, and there are usually exposures that reveal it. Limestone (calcium carbonate) is often a good place to find fossils.