Hey SK-
From another geologist (actually geophysics grad student)... country rock is the rock that an ingenues intrusion intrudes into. So, in CA it would be the rocks that were there before the big intrusions that formed the Sierra's and Yosemite and all the pretty stuff. Does that help? Also, here is a web site with a lot of geology terms defined: http://volcano.und.edu/vwdocs/glossary.html

PABadger