Here's a breakdown of how geological time is organized:

* Eons: The largest division of geological time. Examples include the Precambrian eon and the Phanerozoic eon.

* Eras: Within eons, eras represent significant changes in life forms and geological events. For example, the Phanerozoic eon includes the Paleozoic, Mesozoic, and Cenozoic eras.

* Periods: Eras are further divided into periods, which are marked by specific geological and biological events. Examples include the Jurassic period (Mesozoic era) and the Quaternary period (Cenozoic era).

* Epochs: Periods can be subdivided into epochs, which represent smaller changes within the larger time frame of a period. For example, the Quaternary period includes the Pleistocene and Holocene epochs.

* Ages: Epochs can be further divided into ages, the smallest unit of geological time.

So, to answer your question directly: Eras are divided into periods.