Abstract
Except for normative components, the calculation of which is sufficiently involved to require a separate subroutine, the compositional “derived variables” of chemical petrology may be cast readily in the form of simple arithmetic expressions. With regard to the design of petrographic data bases and their associated information systems, the practical question is whether these should be processed by the base builder, so that the values of each derived variable are incorporated in the base, by an ad hoc subroutine linked to the information system prior to each retrieval or reduction, or by permanent elements of the information system. The third option requires intrinsic capability for decoding and evaluating expressions received as character strings at operation time. Subroutines DKDXPR and EVLXPR provide that capability. Calling program IMDEVL provides a convenient conversational device for testing DKDXPR and EVLXPR.
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