DURING the last decade of rapid development of technique of petroleum geology, problems of sub-surface Stratigraphical correlation have forced themselves to the front, and have engaged the closest attention of geologists operating principally in late Cretaceous and Tertiary oil-fields all over the world. Formerly, and to some extent now, drillers' recognition and classification of rock-chips collected either from bailer or sample box served as a crude guide to underground conditions, though the limited vocabulary and superficial petrological knowledge of the average driller led to some quaint determinations and technically to still more fanciful structural interpretations. ‘Gumbo’ and ‘shell,’ for example, cover a multitude of geological shortcomings, while clay, shale, silt, and sand vary in diagnosis largely according to their degree of wetness when they arrive at the surface; anything productive of white powder to the bit is termed ‘chalk’, and so on. Such casual nomenclature and equally casual sampling has been part of the long-established code of the oil-well driller, and sufficed until the advent of a more exacting petroleum geology signified the impending and much-to-be-desired change.
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MILNER, H. The Stratigraphical Value of Micro-organisms in Petroleum Exploration. Nature 117, 558–560 (1926). https://doi.org/10.1038/117558a0