FIGURE 1.19 (a) Stromatolite built by the trapping and building of sediment particles by microbial communities—1,500 Ma Bil’yakh Group, Siberia. SOURCE: Courtesy of Andrew Knoll, Harvard University. (b) Stromatolites built of seafloor precipitate structures that are composed of calcium carbonate crystals without any obvious templating influence of microorganisms—1,900 Ma Rocknest Formation, Canada. SOURCE: Courtesy of John Grotzinger, Caltech. Used with permission.