Concept of dating history
In Italy, again in the Tuscan Hills in the vicinity of Florence, Giovanni Arduino, regarded by many as the father of Italian geology, proposed a four-component rock succession.His Primary and Secondary divisions are roughly similar to Lehmann’s Primary and Secondary categories.He also described the succession of strata associated with coal-bearing sedimentary rocks in Somersetshire, the same region of England where he had mapped the rock exposures.Johann Gottlob Lehmann of Germany reported on the succession of rocks in the southern part of his country and the Alps, measuring and describing their compositional and spatial variation.According to a long-standing principle of the geosciences, that of superposition, the oldest layer within a sequence of strata is at the base and the layers are progressively younger with ascending order.The relative ages of the rock strata deduced in this manner can be corroborated and at times refined by the examination of the fossil forms present.
As such, they were considered unlikely to recur on what was thought to be an unchanging world.
While making use of Steno’s principle of superposition, Lehmann recognized the existence of three distinct rock assemblages: (1) a successionally lowest category, the Secondary (Flötzgebirge), composed of layered or stratified rocks containing fossils, and (3) a final or successionally youngest sequence of alluvial and related unconsolidated sediments (Angeschwemmtgebirge) thought to represent the most recent record of the Earth’s history.
This threefold classification scheme was successfully applied with minor alterations to studies in other areas of Europe by three of Lehmann’s contemporaries.
age and history of Earth’s rocks and rock assemblages.
Such time determinations are made and the record of past geologic events is deciphered by studying the distribution and succession of rock strata, as well as the character of the fossil organisms preserved within the strata.