Thanks CL for the gift of a new edition of CL's Principles of Geology. JH comments on a number of issues, in a very long letter, including the replacement of extinct species by other species, CL's solution of the problem of the variation of climate, and the evolution of language. JH speculates at length on the isothermal activity under the earth's crust that may result in volcanoes or the formation of new land masses, and describes a considerable number of geological examples that JH sees as supporting CL's ideas.

Ideas expressed by JH in his previous letter to CL (see JH's 1836-2-20) about secular variations of isothermal surfaces of the interior of the earth were similar to ideas expressed by Charles Babbage in a paper to the Geological Society. This was drawn to JH's attention by CL and by R. I. Murchison. JH argues that he did not know of Charles Babbage's paper, and goes on to point out how JH's ideas differ. Concludes with mention of having re-discovered the sixth satellite of Saturn, not seen since its discovery by JH's father, William.