Lewisian
Concise Oxford English Dictionary © 2008 Oxford University Press:
Lewisian /luːˈɪsɪən/
▶adjective Geology of or denoting the earlier stage of the Proterozoic aeon in NW Scotland, when the oldest rocks in Britain were deposited.
– origin C19: from the island of Lewis in the Outer Hebrides (where the chief outcrops of these rocks are found) + -ian.
'Lewisian' also found in these Oxford entries:

