Seriously Misrepresenting The Nominal Group 'Postdeictic'

Martin, Matthiessen & Painter (2010: 168):

In contrast, at the other end of the cline, the Postdeictic is concerned with particular referents; it serves to indicate how particular referents are selected from some general class – for example, particular referents that are the same or different from another set of referents (the same sedimentary rocks, some different sedimentary rocks). 

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This continues the authors' confusion of deixis and reference. Having previously confused the Deictic function in nominal group structure with the cohesive function of demonstrative reference — see previous post — here they confuse the Postdeictic function in nominal group structure with the cohesive function of comparative reference. And again, they confuse the referent (that is referred to) with the nominal group featuring the reference item (that does the "referring).

So, to be clear, the Postdeictic is not concerned with particular "referents"; instead, it identifies a subset of the Thing of the nominal group, as Halliday (1994: 183) and Halliday & Matthiessen (2014: 373) explain:

The post-Deictic identifies a subset of the class of ‘thing’ by referring to its fame or familiarity, its status in the text, or its similarity/dissimilarity to some other designated subset.

So the Postdeictic in the same sedimentary rocks and some different sedimentary rocks identifies a subset of the class of sedimentary rocks.

Importantly, the Postdeictic function is distinct from the comparative reference function of such words, which Halliday & Matthiessen (2014: 632, 633) explain as follows:

In comparative reference, the reference item still signals ‘you know which’; not because the same entity is being referred to over again but rather because there is a frame of reference – something by reference to which what I am now talking about is the same or different, like or unlike, equal or unequal, more or less. Comparative reference items function in nominal and adverbial groups; and the comparison is made with reference either to general features of identity, similarity and difference or to particular features of quality and quantity.

Any expression such as the same, another, similar, different, as big, bigger, less big, and related adverbs such as likewise, differently, equally, presumes some standard of reference in the preceding text.

So the comparative reference items in the same sedimentary rocks and some different sedimentary rocks presume some standard of reference in the preceding text.