OpenLayers 2 is outdated. Go to the latest 3.x version: http://openlayers.org/
- If a Layer is an Overlay, it is not a BaseLayer.
- Overlays (unlike BaseLayers) are not mutually exclusive -- OpenLayers can display many Overlays at the same time (on top of eachother) on a given Map?.
- To hide or show Overlays, the user uses the LayerSwitcher.
- An OpenLayers.Layer? is by default an Overlay (not a BaseLayer)
- For instructions on how to configure an existing Layer as an Overlay, see BaseLayersAndOverlays.