Data includes any facts, samples or statistics that are collected or recorded for later analysis.
His data included primary samples - that is, actual rocks that displayed sedimentary and other geologic traits. Keeping accurate records, descriptions and drawings of these samples also counts as data. In fact, his study was described as "so full of fossils and chemical apparatus that there is hardly room to sit down".
He also undertook field data collection. When rocks were too large to sample (bring back to the lab) or were part of a larger system or structure, field visits were required. Again, recording these visits with scientific observations count as data collection.
In particular were cliffs at Siccar Point - where vertical layers of shale met horizontal layers of bedrock. These cliffs formed some of his data collection that supported his hypothesis of geologic uplifiting.
This is what set his data about the history of earth apart from others and that made his ideas more credible than those of earlier theorists.
Known as the founder of modern geology, James Hutton theories on the history of the Earth were much more credible than some earlier theorists. Hutton believed that the history of the Earth could be understood by how the processes of erosion and sedimentation worked during current times. Hutton's ideas suggested that Earth was continually forming. Sutton is primarily responsible for geology even being recognized as a science. His ideas and approach to studying the Earth established geology as a proper science. Interestingly, Hutton did not seek a career in geology, rather he studied medicine in Europe. Hutton began farming and was fascinated by how weather could impact the crops, which sparked his interest in geology. One key concept that Hutton developed was the Theory of Uniformitarianism, which basically stated that the forces that influence geology today are the exact same forces that happened during past years. This is extremely important, because it gives geologists an idea on how long it would take for certain processes to occur, such as the formation of rocks.