six orders of Chronostratigraphic units, from larger to smaller:
|Units based on numerical assignation (Absolute time)|
|1st Order: Eonothem||Eon|
|2nd Order: Erathem||Era|
|3rd Order: System||Period|
|4th Order: Series||Epoch|
|5th Order: Stage||Age|
|6th Order: Chronozone||Chron|
These units were recognized throughout the work of paleontologist working in Europe for the last three centuries.
In recent years, after the recognition of radioactivity, and radioactive decay, geologist recognized that isotopes could be used to numerically dates, the so-called "absolute time units" the subdivision based on the history of evolution of life on Earth, resulting in the establishment of twin or parallel set of units known as Geochronologic units, these geochronologic units are also referred to as Ďabsolute agesí because they are expressed in numbers, that is, they are the numerical values assigned to the former Chronstratigraphic units.
3.2. Definition of the units based on the evolution of life on Earth:
3.2.1. Eonothem: "Eon" also means any span of one billion years. The largest division of geologic time, including several eras and lasting for hundreds of millions or billions of years. Originally there were only two eons, the Precambrian and the Phanerozoic, because of the long span included in Precambrian, now we recognize two separate eonothems, the Archean (previously Archeozoic) and the Proterozoic.
3.2.2. Erathem: A geologic division that includes several periods, but smaller than an eon. Generally lasts for many tens or hundreds of millions of years, and often characterized by distinct life forms - e.g. the Cenozoic is the "age of mammals". Commonly recognized Erathems are the Paleozoic (ancient life), Mesozoic (middle life), and Cenozoic (new life). Precambrian divisions such as the Proterozoic and the Archean were conventionally eras but are now referred to as eons. New Precambrian eras include the Sinian, the Riphean, the Huronian, etc.
3.2.3. System: Is the most commonly used unit of geologic time, representing one subdivision of an era. Eras may have from two to six or seven systems each. Each system generally lasts for some thirty to eighty million years (the only exception, the mere 1.6 million year long Quaternary, is actually a hold-over from the 19th century formulation, as its numeric name indicates).
3.2.4. Series: A subdivision of a geologic system, a Series usually corresponds to subdivision of the system into a lower, middle and upper part, although the Jurassic System contains Series of special designation such as the Liasic.
3.2.5. Stage: A subdivision of a geologic Series, usually lasting between five and ten million years, although there are also shorter and longer stages. The Stage is the fundamental concept of time in geology, which includes the interval of time represented by the vertical distribution of a particular fossil assemblage at a given geographic locality.
3.2.6. Chron: The smallest subdivision of geologic time lasting only a million years or even less. Usually is defined by the fossil content of a short interval of one species or an assemblage of species.
NOTE: You are
(1) Retaining in the right order (from higher to lower) all the units of the geologic time scale (see your text).
(2) Identifying the origin of each term o name given to the major units of the geologic time scale (System/Perioid and up).
(3) Identifying the original geographic location from where the major geologic units were defined.