not a geologist -
Do not know if this is an overlay or an attempt to show layers or different classifications of the same surface?
This is a folded map. It should be scanned unfolded on the following (or the previous) page. You can always check them on the BHL site (the 2nd icon of a folded page in the classification menu). Please, mark it as No illustration and then hashtag with #folded in the comment.
Thanks for your question!