BY W. N. BENSON. 167 



clearly intrusive into the chert, and containing fragments of 

 ffeliolites, etc. Microscopically, it is entirely crystalline, and 

 consists of shattered and corroded grains of quartz and albite in 

 a finely granular felsitic mosaic. One may also recall the brec- 

 ciated keratophyre that passes into calcareous agglomerate near 

 Silver Gully. Another significant feature is the almost entire 

 absence of glassy matter from these "tuffs" and "breccias," and 

 the frequency with which fragments of -keratophyre, and even 

 magnetite-keratophyre occur in them. They have been found in 

 the " tuffs " of the Moonbi, Attunga, Manilla, and Bingara dis- 

 tricts, and also in the Baldwin Agglomerates. Though, at first 

 sight, the term "intrusive tuff" may seem a contradiction in 

 terms, yet intrusion-breccias are well known, and considerations 

 similar to tliose explaining the close relation of intrusion and 

 extrusion in suboceanic vulcanicity (see p. 124) may assist in the 

 explanation of this apparent anomaly. 



Further evidence from the field and laboratory is necessary, 

 before these rocks can be profitably discussed. 



Summary and Acknowledgments. — The spilitic series of erup- 

 tions in the Nundle district included spilites, dolerites, and 

 keratophyres. So far as can be seen, they are all intrusive into 

 the sediments, and certain spilites intrusive into soft muds, 

 have produced pillowy masses. They are nearly all rich in 

 albite, which appears to be chiefly primary.. They do not show 

 at all clearly the evidence for the secondary character of the 

 albite described by Messrs. Flett and Dewey, or that noted by 

 Termier ; nor is there evidence that the soda-content of the 

 magma has been segregated in the manner discussed by Daly. 

 Magnetite-keratophyres occur, and their development was brought 

 about by magmatic differentiation assisted by pneumatolysis. 

 Many of their features recall the magnetite-syenite-porphyries of 

 Lappland. An attempt is made to explain their varied struc- 

 tural features by a consideration of magma-viscosity. The forma- 

 tion of ferruginous jasper-veins is described as a post-volcanic 

 process. No complete account can yet be given of the mode of 

 eruption of the rocks, and, in particular, of the manner of forma- 

 tion of the associated breccias. 



