Choose two of the four major divisions in the geologic time scale, describe each one and explain one way in which they were different and one way in which they were similar.

Two of the four major divisions in the geologic time scale are the Archean eon and proterozoic eon.

Explanation:

The Arcean eon is dated from 3900 million years ago to 2500 million years ago. It was in this period the oldest known rocks were deposited.  At that time the atmosphere of earth was composed of  gases like methane, carbon monoxide, ammonia etc.  

The Proterozoic eon is dated from 2500 million years ago to 540 million years ago. It was by the end of Proterozoic era that the Eukaryotas came into existence.  

There are some similarities and differences between Archean eon and Proterozoic eon.  Stromatolites formed by the colonies of cyanobacteria lived in both the eras. This is one similarity between both the eras.  

The weather in the Archean era was hot and wet while the climate and weather of proterozoic era was different from this. Proterozoic era experienced two ice ages.