114: Transactions. 



of 6 in., while the largest are as much as 10 ft. across. The material that 

 separates the pillows consists of fragments of pillows that have become 

 broken lip ; some of the pieces are tachylyte, others have a selvage of 

 tachylyte, while others are free from tachylyte ; and this interstitial 

 material is cemented by crystalline calcite. Only in one place does fossil- 

 iferous limestone fill the interspaces, and that is in the neighbourhood 

 of a " limestone dyke," seven or eight of which have penetrated the vertical 

 fissures in the lava. As limestone occurs above the rock, it is more than 

 probable that in this case at least the separating material has come from 

 above. Boulton (1904, p. 158), in describing British pillow-lavas similar 

 in many respects to the present rock, was of the opinion that the limestone 

 came from below. 



In places the fragmentary matter makes up most of the rock, with a 

 scattered pillow here and there; elsewhere pillows are massed together and 

 the group is isolated in a matrix of the finer tufaceous-looking material, 

 while the greater portion of the pillows are grouped in a fashion similar 

 to the lower pillow-lava. 



The occurrence of f ossiferous limestone between the pillows at Oamaru 

 Cape, together with the fossiliferous tuffs and limestones above and below 

 the lower lava, undoubtedly point to eruption under submarine conditions. 

 The glassy selvage which is everywhere present indicates rapid cooling of 

 the masses, and this would take place in contact with water. The differ- 

 ences in the structure of the two rock-masses just described evidently point 

 to some difference in the mode of eruption. Reid (1907, p. 51) says, " It 

 is still a moot point whether pillow-lavas are true outflows or are intruded 

 sills." Tempest Anderson (1910, p. 632) witnessed the formation of the 

 pillows as the lava entered the water at Savaii. Geikie (1897) and Teall 

 (1899) ascribe the structure to intrusion into loosely compacted sediments. 

 Benson (1915, p. 125) recognized intrusive contacts with the surrounding 

 sediments in the Nundle district, New South Wales. In the present case 

 the lower lava may be intrusive, but there is no positive evidence to 

 support this view, except the fact that there is no sign of explosive action 

 in the mass, as there is in the upper lava. The indications in the field 

 strongly suggested that the latter rock after coming into contact with the 

 water, and after the invidualization of the pillows, underwent disintegration 

 through local explosions in the mass of the flow, the resulting fragments 

 then settling down through the water and becoming incorporated in the 

 main mass as it flowed over the sea-bottom. After cooling, the rock became 

 fissured, and the subsequently deposited calcareous mud penetrated and 

 filled the cracks, forming dyke-like masses. Pillow-lavas, although fre- 

 quently occurring as deep-sea lava-flows, are not restricted to conditions of 

 great depth, for Tempest Anderson observed the lava flowing into the sea 

 at Savaii. The nature of the sediments above and below the upper lava 

 at Oamaru clearly indicate shallow-water conditions. In fact, there is 

 reason to believe that the rocks of the Oamaru series were all deposited in 

 comparatively shallow wa'.er. 



VII. Chemical and Petrographical Notes. 



Some preliminary work has been done on the microscopical characters 

 of these pillow-lavas, and chemical analyses made of one of the freshest 

 types from Awamoa Creek near Deborah. More detailed field work is 

 necessary before a full account can be given of these rocks. 



