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Several geologists have drawn variation diagrams to demonstrate a continuous gradation in chemical composition between all members of the anorthositic and the quartz mangerite series and have inferred that this is a major support for the hypothesis of comagmatic origin. The alternative hypothesis elaborated here is that the plots of major oxides against differentiation indices are discontinuous and can be interpreted in terms of two independent magma series, each gradational throughout, but with contrasted trends of differentiation. A gabbroic anorthositic liquid may be generated in the lower crust and a quartz mangeritic liquid successively later in a relatively dry deep part of the upper crust, possibly by development of anatectic granitic liquid with concomitant reaction with and differential assimilation of appropriate constituents of associated undersaturated metagabbro.

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