Lively debate.

In "Martian Invasion" (SN: 11/8/03, p. 298), the white cliffs of Dover are offered as a "notable example" of the precipitated carbonate deposits some have expected to find had Mars been wet and warm in the past. The Dover chalks are an unfortunate choice for comparison because they're composed primarily of the calcitic remains of microscopic marine phytoplankton. As most carbonate sediments on Earth are the result of direct or indirect biogenic activity, extensive examples of inorganically precipitated carbonate sediments will be hard to find.

K.M. TOWE, TENNILLE, GA.