When you select the Geology Tab, you will find that Geology off is highlighted. Below Geology off, you will see Classify by: followed by three selections.
When you select one of the three available choices, the associated interactive geology map will overlay your current map selections. Hover your mouse over any feature, and the geology data will be provided via a pop-up box.
In the example below, we selected the crater, Marius, to show the data for all three geological selections.
When you are done studying geological features, you can de-select Geology by highlighting Geology off once again. The program will also default to Geology off whenever it is shut down.
A complete Geology Legend for all three map formats is available in the Help Menu.