The cause of the assumption was the proximity.

Passage:

Archeologists who discovered and excavated the small cave in 1983 had assumed that the brittle black material on the skulls was asphalt because of its color and the site's proximity to the largest asphalt deposits in Isreal.

Represented so far

An Archeologist who discovered and excavated a small cave has assumed the brittle black material on a skull was asphalt because of the cave's proximity to a large asphalt deposit.

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