GEOLOGY. — The great naturalist and philosopher Ulisse Aldrovandi is credited with introducing (1603) the term *geologia* (from γῆ, earth, and λόγος, discourse, treatise, history). From the vast field of doctrines that this term originally encompassed and long retained, geology gradually narrowed its boundaries, particularly during the 19th century, as certain parts broke away to form new branches of science. However, the clear and definitive delimitation of geology’s competence in its strictest sense—pure geology—is even more recent.
Assuming knowledge of rocks (lithology, petrography) and fossils (paleontology), geology may be defined as the science that primarily studies the structural characteristics of the Earth’s crust (lithosphere), namely the conditions of development and mutual position (attitude) of rocks of varying nature and origin that form the crust itself, as well as the processes and causes that have led to its present configuration and reflect the events the Earth has undergone during its vast geological ages.
It would, however, be impossible to reconstruct these events in any way other than excessively incomplete and often entirely erroneous, relying solely on the elements derived from the nature, distribution, and mutual attitude of rock formations.
To achieve this goal, only fossils—remains of plants and animals that lived in past ages—can provide the indispensable documentation. These remains, found within sedimentary rocks of all geological ages and often in a remarkably preserved state, allow for the reconstruction of the characteristics and sequence of vanished faunas and floras, as well as the physical environment in which they lived. They also furnish elements for depicting the various natural features of the phenomena that, in a subsequent phase, progressively modified the features of our planet.
The studies and investigations that lead or aim to lead to the objectives just mentioned constitute the essence of geology in its narrowest and most genuine sense. Thus delimited, geology comprises two fundamental parts: structural geology and historical geology, also respectively designated by the terms *tectonics* and *stratigraphy*.
The former deals with the most varied ways in which rocks are arranged within the complex structure of the lithosphere, identifying their most characteristic types with reference to the three categories into which rocks are grouped based on their origin: igneous and eruptive, sedimentary or stratified, and metamorphic.
Historical geology or stratigraphy has as its main objective the reconstruction of the history of our planet by determining the relative age of the rocks that form the framework of the Earth’s crust.
It may be said that there is no science, beyond the basic ones—petrography and paleontology—from which geology does not derive some assistance for its objectives or the hypotheses on which its theories and reconstructions are based: from physical chemistry to oceanography, from zoology to volcanology, from geochemistry to applied geophysics, etc. It is therefore an eminently synthetic science. On the other hand, geology forms the foundation of many other branches of science, some of which were once part of it and later developed to such an extent as to become autonomous entities. For example, geomorphology, volcanology, glaciology, applied geology, and many others.
For the historical-scientific development of the three fundamental parts of geology, see PALEONTOLOGY; STRATIGRAPHY; TECTONICS.