| environ. |
chalk A soft, pure, earthy, fine-textured, usually white to light gray or buff limestone of marine origin, consisting almost wholly (90-99%) of calcite, formed mainly by shallow-water accumulation of calcareous tests of floating microorganisms (chiefly foraminifers) and of comminuted remains of calcareous algae (such as cocoliths and rhabdoliths), set in a structureless matrix of very finely crystalline calcite. The rock is porous, somewhat friable, and only slightly coherent. It may include the remains of bottom-dwelling forms (e.g. ammonites, echinoderms, and pelecypods), and nodules of chert and pyrite. The best known and most widespread chalks are of Cretaceous age, such as those exposed in cliffs on both sides of the English Channel (一种柔软、纯净、像土壤一样、细质地的、通常从白色到浅灰色或海相成因的黄色的石灰石，组成几乎全部(90-99%)是方解石，主要形成过程是通过在浅水中的碳酸钙的积累，浮游微生物(主要是有孔虫)和石灰藻的粉碎的遗体(如小円石和棒石)的合成，形成很细的晶状方解石的无定形模型。石块是疏松的，有些易碎的，只是略微一致。它可能包括的底栖的形式(例如，菊石类、棘皮动物、双壳类)的遗体，以及燧石和黄铁矿的节结。最著名的和最广泛的白垩产生于白垩纪，例如那些在英吉利海峡的悬崖离岸边发现的白垩。) |
