Georgius Agricola (Georg Bauer), also known as the 'father of mineralogy', is considered the founder of geology as a scientific discipline. That is, he provided the foundations for the study of the Earth (and its rocks, minerals, and fossils), in a systematic, recorded, way.
Whilst practising medicine, Agricola also spent a lot of time studying and observing mining operations. The resulting text (De Re Metallica) was devoted not only to studies of the ores and minerals, but also to technology and the 'best practise' methods of the time. Compared to previous texts, the De Re Metallica was founded on scientific tests and observations, rather than speculation.
De natura fossilium is considered the first book of mineralogy. This text presented the first classification of minerals (which was based on their physical properties), as well as laying the foundations for studies of stratifications. The work also presented a summary of the Ancient Greek and Roman texts on the subject, including notes where present day observations contradicted the earlier works.
As well as his work on mining geology and mineralogy, Agricola also published the first book on physical geology (De Ortu et Causis Subterraneorum). This book is notable as it included oceans and wind as powerful geological forces. Furthermore, it provided an explanation for volcanic eruptions and earthquakes: subterranean gasses warmed by the Earth's core.