James Hutton could be said to be the first modern geologist. In 1785 he presented a paper named Theory of the Earth to the [Royal Society of Edinburgh]?, in which he explained his theory that the Earth must be much older than had previously been supposed, to allow enough time for mountains to be eroded and the sediment to form new rocks at the bottom of the sea, which were then raised up to dry land.
Followers of Hutton were known as plutonists because they believed that some rocks were formed by volcanoes from lava, as opposed to the neptunists, who believed that all rocks had settled out of a large ocean that gradually lowered over time.
[William Smith]? (1769-1839) drew some of the first geological maps and started to put rock strata (layers) in order by using the fossils in them.
Sir Charles Lyell started to publish his famous book, Principles of Geology, in 1830 and continued to publish new revisions until he died in 1875. He successfully promoted the doctrine of uniformitarianism, the belief that geological processes happen slowly over time and are still happening today, as opposed to catastrophism, the belief that the earth's features were formed in catastrophic events and are unchanged between them. (Hutton believed in uniformitarianism but it wasn't widely accepted at the time.)
The idea of continental drift was promoted by [Alfred Wegener]? starting in 1912 and by Arthur Holmes, but wasn't accepted until the 1960's when the theory of plate tectonics was developed.
There are many different fields within the discipline of Geology, and it would be hard to list all of them. Some include, however, Hydrogeology? (or Geohydrology?), [Petroleum geology]?, [Soil science]?, Climatology?, Biogeology?. Subdisciplines within geology proper include structural geology, [sedimentology and stratigraphy]? (Sed/Strat?), mineralogy (study of minerals), petrology (study of rocks), geomorphology (study of landforms), seismology (also a field in geophysics?). There is also [engineering geology]?, which supports civil, especially geotechnical engineering, and [geological engineering]?. The difference between geological engineering and engineering geology is real: geological engineers are licensed as engineers, engineering geologists are licensed as geologists.
See also geologists, the Geologic Timescale, Minerals
James Hutton's Theory of the Earth: http://www.mala.bc.ca/~johnstoi/essays/Hutton.htm
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