In contrast to the Proterozoic, Archean rocks are often heavily metamorphized deep-water sediments, such as [[graywacke]]s, [[mudstone]]s, volcanic sediments, and [[banded iron formation]]s. [[Carbonate]] rocks are rare, indicating that the oceans were more acidic due to dissolved [[carbon dioxide]] than during the Proterozoic.<ref>John D. Cooper, Richard H. Miller, and Jacqueline Patterson, ''A Trip Through Time: Principles of Historical Geology'', (Columbus: Merrill Publishing Company, 1986), p. 180.</ref> [[Greenstone belt]]s are typical Archean formations, consisting of alternating units of metamorphosed [[mafic]] igneous and sedimentary rocks. The meta-igneous rocks were derived from volcanic [[island arc]]s, while the metasediments represent deep-sea sediments eroded from the neighboring island arcs and deposited in a [[forearc|forearc basin]]. Greenstone belts represent sutures between protocontinents.<ref>Stanley, pp. 302-3</ref>
Fossils of cyanobacterial mats ([[stromatolites]]) are found throughout the Archean, becoming especially common late in the eon, while a few probable [[bacterium|bacterial]] [[fossils]] are known from [[chert]] beds.<ref>Stanley, 307</ref> In addition to the domain [[Bacteria]] (once known as [[Eubacteria]]), microfossils of the domain [[Archaea]] have also been identified.