page 222 note 1 The data thus brought to bear upon the problem of the genesis of these ilmenite porphyroblasts, correlated with corresponding data obtained in an investigation of pyritous slates, particularly those from Penrhyn slate quarry, Carnarvonshire, suggest a somewhat similar mechanism for the genesis of porphyroblastie pyrites and epidote: when reconstitution-trend is in the direction of chlorite, the residual ehloritie matter tends to become enriched in ferrous oxide; and if sulphates are also undergoing reduction to sulphides, porphyreblasts of pyrites may develop. Magnesia thus liberated would become an increment available for evolving chlorite. But lime is not markedly assimilable to chlorite, and like ferrous oxide it tends to be relegated to the residual chloritic matter; in the latter, a hydrous system, it may react with ferrous oxide and triad-oxides to form epidote:
In the case of the Calvario phyllite, titanite may conceivably develop in preference to ilmenite at some loci, and ilmenite constituted at a stage when the limeconcentration in the chlorit, ic matter was low may become liable to alteration to titanite as the lime-concentration increases.
The wedges and prisms of quartz within the selvage of the ihnenite crystals described, and similar quartz structures about porphyroblasts of pyrites (e. g. in the Penrhyn slate), are in the main disposed normal to the crystal wall or matrix wall of the selvage: they recall the structures assumed by quartz crystals sessile on a cavity wall. A reduction of volume is achieved by the chemical combinations yielding porphyroblastic ores; tendencies towards a similar reduction of volume are operative around foci of re-crystallization in the groundmass and may continue to be operative after ores have separated. It is suggested that tension within the groundmass may partly detach the latter from the ores and tend to produce around them a cavity which is ultimately infilled with quartz. It is proposed to discuss this phenomenon more generally in a later paper.
