(Jacksonville, OR)  --  The man credited with inventing the "pet rock" is dead at age 78.  Gary Dahl, who grew up in Spokane, died on March 23rd in Jacksonville, Oregon, following a series of health problems.  Dahl's daughter says he made the pet rocks for a dollar apiece and sold them for four dollars each during their heyday in the mid-1970s.  Dahl once said the idea came to him after hearing people complain about their pets and he began marketing a "pet" that needed no care.