The Pony Rider Boys in Montana by Frank Gee Patchin

Yee-Haaw! The Pony Rider Boys are on the move again! In this book, the 3rd of the series, the boys have decided that they want to explore the north country. They also want to make their own arrangements for the adventure, with the approval of Professor Zepplin, of course! So they have arrived in Forsythe, Montana, to try their luck in the mountains.