The complementary agent explains who performed an action. The cyclical nature of geological changes was recognised by Hutton. Hutton published Theory of the Earth with Proof and Illustration in 1785. Theory of the Earth with Proof and Illustration was published in 1785 by Hutton.
Igneous intrusions produce hornfels.
Hornfels are produced by igneous intrusions . The agent is not expressed when unknown or when it is unnecessary. In the first and third examples the agent is necessary. In the second example it is not as it would be sufficient to know the name of the book and publication. The Theory of Uniformitarianism was proposed by Hutton .