Final answer:
James Hutton first suggested the concept of uniformitarianism, later which was fully developed and popularized by Charles Lyell, making Lyell the founder of the principle as known in modern geology.
Step-by-step explanation:
The concept of uniformitarianism was first suggested by James Hutton, a Scottish geologist, in the 18th century. He proposed that Earth's geological features could be explained by slow and consistent processes over time, such as erosion and sedimentation. This idea was in stark contrast to the then-popular beliefs in catastrophism. Hutton's theories laid the groundwork for modern geology and were later expanded upon and popularized by Charles Lyell, whose work significantly influenced Charles Darwin's theory of evolution. Lyell emphasized the consistency of geological processes and concluded that the Earth had to be much older than the few thousand years suggested by biblical chronology.
Therefore, while Hutton is known for originating the idea, it was Lyell who is credited with the founding of uniformitarianism as a fully developed principle in geology. His seminal work, Principles of Geology, explained that the slow-acting natural forces in the past were the same as those observable in the present, thus adhering to the maxim that 'The present is the key to the past'.