Mark and Guy:
Based on the photographs—and particularly the first one—it appears the power source, probably steam or perhaps a gasoline engine, sat down in the lower portion of the structure and the drive belt came up through the relatively narrow opening aligned with the one side of the machinery foundation. I suspect the sawmill had a least a roof over it. I will be most interested in whatever else you find out about it. It is a very permanent installation, meaning the lumbermen pulled the timber to the mill, and it might just be one of the largest more modern sawmills of its kind in the Pines.
Best regards,
Jerseyman