Lyel's theory that geological processes occurred over time led Darwin to formulate what concepts?
In a way, natural selection.
Lyell gave Darwin the idea that organisms do not simply mutate instantaneously as other biologists had theorized, but that populations actually change over long periods of time.
The prevailing biological theory proposed by Jean-Baptiste Lamarck was that organisms acquired traits over their lifetime and then passed them on to their kids. For instance I worked out a lot and put on a ton of muscle and then had kids, my kids would naturally have a lot of muscle.
Lyell's ideas made Darwin doubt this theory and pursue another line of thinking that led to his proposal that natural selection occurs not on the organismal level, but rather at the population level, and occurs over numerous generations.
Source: I've read a lot about Darwin.