The gross heathenism of civilization has generally destroyed nature, and poetry, and all that is spiritual.

In this line Muir condemns what he saw as the spiritually deadening effects of industrial “civilization.” By calling it “gross heathenism,” he reverses the usual civilizational hierarchy: modern society, despite its claims to refinement and progress, behaves like a crude idolater—worshiping material gain while trampling the sacredness of the natural world. The triad “nature, and poetry, and all that is spiritual” links ecological destruction to cultural and inner impoverishment: when landscapes are degraded, the human capacities for wonder, imagination, and reverence are also diminished. The sentence reflects Muir’s characteristic fusion of environmental critique with moral and quasi-religious language.