Stone and Wallis (1951. 113) introduced Group Ia as:
'Microscopically, the augite is not quite completely altered to hornblende, and the feldspars are not so extensively riddled with small needles of hornblende. The hornblende often occurs in small blades showing somewhat parallel arrangement. In other respects sections are very similar in character to the well known Group I, though nothing like so homogeneous.'
Seventeen Group Ia thin-sections were examined and, following the process described above, six petrographically recognisable sub-groups have been recognised.
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Last updated: Wed Jul 29 2009