The Metamorphic Belts

To show the relation of the Trent River area to the schist belt and the Alpine Fault, a sketch map (fig. 2) is presented on a scale of 4.3 miles to an inch of the whole of Sheet S52 (Harpers Pass). The Alpine Fault crosses the north west corner of this sheet. Although the geology of the whole sheet is shown, only the schists and Triassic rocks on the south-east side of the fault are described. These rocks are divided into five belts numbered I to V in order of increasing metamorphism. It is assumed that these belts increase in age with increasing metamorphism, but only in the less metamorphosed belts can this be proved by internal evidence.

Metamorphism progressively decreases away from the Alpine Fault; no metamorphic jumps are known and metamorphic belts can be mapped only by accepting rocks of specific ranks as marking their limits. Rocks of these specific ranks can be recognised at different places without much difficulty if care is taken to compare rocks that were originally similar. It is less easy to describe these rocks so that other geologists can recognize them. Hutton and Turner (1936) have set up standards for a limited range of low-rank schists, and other petrologists had earlier set up standards based on index minerals for the higher-rank metamorphic rocks with which this paper is not concerned. But none have been proposed for the sub-schistoze rocks which occupy the bulk of the area described. For this reason we regard our boundaries as tentative and subject to modification if and when such standards are set up.