The Pony Rider Boys in the Grand Canyon - Frank Gee Patchin

Yee-Haaww! The Pony Rider Boys are on the move again. The boys are back home, but as they are chopping wood, it is decided that they need a new adventure out west. Mr. Perkin's, Walter's dad, has suggested the Grand Canyon. So, meeting Professor Zepplin on the way, they set out on the train for Arizona. (Summary by Ann Boulais)

Previous book in the series: The Pony Rider Boys in New Mexico
Next book in the series: The Pony Rider Boys with the Texas Rangers