Karen, In the book I quoted from is this explanation: "1850, Clackamas, Marion and Linn extended by statute to the Rocky Mountains, but the 1850 census population lay west of the Cascade Mountains." I live in Wasco county. It covered four states including Idaho, Montana and Wyoming. Wasco county was the largest county in the U.S. People unaware of its history would not know to contact the governmental offices here in Wasco county to search for records of their ancestors in newer counties that were formed later. The records are not sent to the newly formed counties, they remain with the original counties. In 1860 Idaho was considered a part of the "Washington Territory". It became "Idaho Territory" around 1870. By 1870 Wasco county had been downsized and the counties of Umatilla, Union, Grant and Baker were formed in eastern Oregon.