Smaller communities often use the post offices of nearby cities and no special indication (other than perhaps the return address as in this instance) that they originated from outside the city is evident in the postmark. There are occasions however when outlying facilities had their own canceling device even though the mail was routed through the larger post office.
In this case, the postmark reads “Mayville Rur. (rural) Sta. (station)” along the bottom of the datestamp in addition to the standard town name of “Fossil, Oregon” along the top. The post office began operation in October 1884 with Samuel Thornton as postmaster. I’ve been unable to find the date of final operation for this station in the literature I’m familiar with.