Deposits of Organic Origin

Abstract

In this chapter are treated those secondary deposits which are due, directly or indirectly, to the vital activities of animals and plants. These materials accumulate mainly on the sea floor, but fresh water and terrestrial examples are also well known. Only in rainless deserts and in the frozen polar lands are they absent. A rock of organic origin may be built up directly, from the beginning, as a quite solid material, as in the case of coral rocks and some algal limestones.¹ In other cases the deposition may be biochemical or biomechanical; biochemical, when the vital activities of the organisms promote chemical conditions which favour precipitation, as in the cases of bacterial iron ores and limestones; biomechanical, when the rock is due to the detrital accumulation of organic materials, as in the cases of crinoidal and shelly limestones, and some coals. It is obvious that, in this case, there may be gradual transitions to sedimentary types through an increasing admixture of inorganic detrital materials. In like manner biochemical deposits may pass into rocks of purely inorganic chemical origin. In many instances it is difficult to decide whether a given rock should be assigned to the sedimentary, chemical, or organic groups.