Taking full advantage of the instruction he received, he was able to support himself by teaching, and to continue his studies independently.

Buffon's Theory of the Earth interested him, and in 1753 he successfully competed for a prize by writing an essay on the ancient connection between England and France.

In 1763 he made observations in Auvergne, recognizing that the prismatic basalts were old lava streams, comparing them with the columns of the Giant's Causeway in Ireland, and referring them to the operations of extinct volcanoes.

As remarked by Sir Archibald Geikie, the doctrine of the origin of valleys by the erosive action of the streams which flow through them was first clearly taught by Desmarest.

An enlarged and improved edition of his map of the volcanic region of Auvergne was published after his death, in 1823, by his son Anselme Gaëtan Desmarest (1784-1838), who was distinguished as a zoologist, and author of memoirs on recent and fossil crustaceans.