County supervisors will vote next week on a project that would demolish a great hunk of northern L.A. County‘s remaining natural habitat.If they reject the long-delayed Transit Mix Concrete gravel-digging project, supervisors will stand with its fastest-growing and third largest city, Santa Clarita, which is suing several federal agencies for failing to protect rare species from the mining proposal.The threatened area is near Soledad Canyon, an undeveloped and scenic valley of sage and woodlands two miles east of Santa Clarita. The city’s suit accuses the Department of the Interior and its subsidiaries, the U.S. Bureau of Fish and Game and the Bureau of Land Management, of failing to protect three endangered species: the three-spine unarmored stickleback fish, the Southwestern arroyo toad and a scarce plant called the slender-horned spineflower.