Abstract Derivation: Greek alios , other; khthon , earth, ground. Definition: An allochthonous rock mass or allochthone is a rock unit which has been transported from its original site of emplacement. History and usage: ( a ) First used as an adjective to designate rocks of distant origin: allochthone limnatische Gesteine (Naumann, 1858, p. 657). Gümbel (1884) applies the term to coal formed by transported plant material (hence, allochthonous coal). ( b ) At the beginning of the 20th century, the term acquired a tectonic connotation to designate rock units of varying dimensions (such as nappes and nappe complexes) which, as a result of tectonic transport, have come to rest on a distant substratum. The term still is being used in sense ( a ) for transported coal and other transported residual rocks, and in sense ( b ) for transported tectonic units ( e.g ., Kay, 1945 b, p. 440). It is, however, commonly difficult to determine whether transport has been tectonic or sedimentary, particularly in the case of very small constituents, also known as exotic blocks ( e.g ., in the Wildflysch). Some authors use allochthone to refer to various redeposited sedimentary materials from more or less distant sources (usage of some Italian authors). This extension of meaning should be limited.

Abstract This volume represents a cooperative scientific effort by the International Geological Congress, the National Science Foundation and the AAPG in creating this attempt at standardication of geologic terminology. Providing a guide to past and current usage, and making recommendations where current usage is ambiguous or might lead to mis-understadings, the publication is 196 pages of annotations and extensive bibliography.