Stratigraphy, the study of strata, allows us to date the age of rocks and fossils found in rocks in a way. For this purpose we will use the following principles:

• The law of superpostition: The lower strata are in general OLDER than the upper strata.

• The principle of FLORAL AND FAUNAL succession: The fossils of living things in the lower strata are OLDER than the fossils in the upper strata.

• The Principle of continuity: A stratum is approximately, the same age along all its LENGTH.
• Principio de identidad PALEONTOLÓGICA : Dos conjuntos de estratos que tienen fósiles idénticos son de la
misma edad.
La datación basada en estos principios recibe el nombre de ABSOLUTA , pues no permite conocer la
edad real de las rocas y sus fósiles sino únicamente aventurar cuales son más antiguas y cuáles más modernas.
Como se basa en los estratos únicamente podremos datar las rocas ESTRATIGRÁFICAS y metamórficas.

The principal index or dating fossils are:
• index fossil in the primary era (570 m.y to 65 m.y) .
• index fossil in the secondary era (230 m.y. to 65 m.y.).
• index fossil in the terciary era (65 m.y to the present day)

Index fossils are characteristic of a certain age and so they are very useful because they allow us to the in which they are found.
The characteristics of an index fossil are the following:
- They lived in a relatively time period.
- They have a wide distribution.
- They are very abundant, because they easily.

The importance of fossils

- From a biological point of view, fossils are important because they
allow us to know how the process of living things happened.

- From a geological point of view, fossils are important because they allow us to know the of the rocks in which they are found.

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The Earth's average temperature is ºC; This is one of the factors that makes the existence of life possible. This temperature is due in part to the effect, which exists thanks to the presence of gases such as carbon dioxide or in the atmosphere.
Human beings have changed this balance. Since the the amount of atmospheric CO2 has increased considerably because of the huge use of fuels.

The fossils exclusive or characteristic of a particular epoch are called fossils.
is the study of the age of trees analysing the annual growth rings of their trunks.
methods are based on the spontaneous desintegration of unstable radioactive isotopes that change into other more stable elements.
are layers of sediments produced in glacial lakes alternating between clear and dark layers.

, the study of strata, allows us to date the age of rocks and fossils found in rocks in a way. For this purpose we will use the following principles:

• The law of superposition : The lower strata are in general than the upper strata.

• The principle of succession: The fossils of living things in the lower strata are than the fossils in the upper strata.

• The Principle of continuity: A stratum is approximately, the same age along all its .
• Principle of . Two different layers that have the same fossil are from the same age.
The data based on these principles is called , because it allows us not only to know the real age of rocks and fossils but also to estimate which are older and which are more recent.

This type of data, based only on the existing strata, only allows us to date AND metamorphic rocks.