what is the primary basis for differentiating the eras?

Since this is not my area of expertise, I searched Google under the key words "differentiating geological eras" to get these possible sources:

http://www.google.com/search?client=safari&rls=en&q=differentiating+geological+eras&ie=UTF-8&oe=UTF-8

In the future, you can find the information you desire more quickly, if you use appropriate key words to do your own search. Also see http://hanlib.sou.edu/searchtools/.

The primary basis for differentiating eras typically depends on the context or subject being discussed. In various fields such as geology, history, art, and literature, different criteria are used to categorize and distinguish one era from another. Here are a few examples:

1. Geology: In geology, eras are categorized based on significant changes in the Earth's geological history, particularly related to the types of rock formations, fossil records, and major geological events. The three primary eras in geological time are the Paleozoic, Mesozoic, and Cenozoic Eras.

2. History: Historically, eras are often defined by major political, social, and cultural changes that influence an entire era or period of time. For instance, in European history, eras such as the Renaissance, Enlightenment, Industrial Revolution, and World Wars are commonly recognized due to their profound impacts on society and the economy.

3. Art: In art, eras are distinguished by distinct artistic styles, techniques, and themes prevalent during a specific period. For example, the Renaissance era in Europe (14th-17th centuries) is known for its focus on humanism, realism, and the revival of classical art forms, while the Abstract Expressionism movement in the mid-20th century is characterized by non-representational and expressive artworks.

4. Literature: In literature, eras are identified by prevailing literary styles, themes, and movements of a particular time period. For instance, the Elizabethan era in English literature (late 16th century) is associated with the works of William Shakespeare and other Renaissance playwrights, while the Harlem Renaissance era (1920s-1930s) produced significant contributions from African-American writers and artists.

So, depending on the field of study or the subject matter at hand, the primary basis for differentiating eras will vary accordingly.