BY W. N. BENSON. 569 



which we can assume for the sea in which the sediments were 

 deposited, would give an overburden of water and silt sufficient 

 to act in the manner indicated. There would also need to be a 

 rather nice adjustment of the rate of protrusion of the magma. 

 If it were greater than that necessary for the development of 

 intrusive tuffs, masses of lava would invade the sediments, and 

 different features would result. 



It seems possible to explain thus the passage between the 

 massive and brecciated igneous rocks seen on East Gap Hill, and 

 also, though not so clearly, on the ridge north-west of Tintinhull 

 Hail way-platform. The upper portion of a mass of lava intrusive 

 into, or flowing through wet, semi-consolidated sediment, would 

 naturally be especially liable to brecciation. Again, the abundant 

 opportunity offered by such brecciation for the passage of solu- 

 tions would be exceptionally favourable for the destruction of 

 the ferromagnesian minerals, and the oxidation of their iron- 

 content to magnetite and haematite. To some such processes, 

 the peculiar features of the red breccias and agglomerates of 

 East and West Gap Hills may owe their origin. 



The sections exposed in the railway-cuttings between Tam- 

 worth and Tintinhull, particularly those immediately east of 

 Nemingha, afford further examples of this phenomenon, as will 

 be seen from the features illustrated in the figures herewith. 

 (Text-figs. 6, 7, 10, 12).^ An intrusion of pyroclastic material into 

 partially consolidated sediment might be expected frequently to 

 transgress the bedding-planes of the sediments, to ^rrumple them, 

 and to include numerous crumpled or uncrumpled fragments of 

 them. The exposures illustrated clearly exhibit these features, 

 and further indubitable evidence of intrusion is afforded by the 

 microscopical preparation illustrated in Plate liii., fig. 6. The 

 original of this is in the collection of the Geological Survey 

 of New South Wales (No. 1190), and, was one of the slides used 

 by Messrs. Pittman and David in the preparation of their work. 

 The writer is indebted to these gentlemen for permission to 



* Compare with these illustrations the figure given by Sir A. Geikie of a 

 breccia invading a slate (19, Vol. ii., p.50, fig.198). The explanation of 

 this feature is not, however, applicable to the Tamworth rocks. 



^l^^^X"^' 



