Uttley. — Volcanic Rocks of Oamaru. 113 



(6.) Some of the limestones are not true coral-reef limestones, but. 

 according to Sir John Murray, are " chiefly made up of cal- 

 careous organisms, fragments of molluscs, echinoderms. Polyzoa, 

 and calcareous algae, together with a large number of Foramin- 

 ifera." 

 The plutonic rock mentioned above is a gabbro. Garnet also occurs, as 

 it does in the Oamaru rocks. The intrusive rocks, however, are augite and 

 hypersthene andesites. 



In view of Mr. Suter's remarks in regard to the genus Pyrula, mentioned 

 above, it is interesting to find that this genus occurs as a fossil in the 

 tufaceous rocks of the island of Mango, one of the Tonga Islands. 



VI. The Pillow-lavas. 



Park (1905, p. 513) was the first geologist to recognize the lower rock 

 near the breakwater as a pillow-lava. Since then the present writer has 

 discovered the same peculiar rock in other localities of the district — in the 

 basin of Oamaru Creek, and in the Awamoa Creek near Deborah. At the 

 breakwater, also, the upper so-called " tachylyte breccia " is undoubtedly 

 a pillow-lava which in parts has become brecciated, probably through local 

 explosive action when coming into contact with the sea-water. 



The lower pillow-lava (see fig. 1) consists mainly of spheroidal masses 

 of lava with the interspaces filled Math fossiliferous limestone. The junction 

 with the tuffs below is quite even, and the surface of the tuft's shows no 

 irregularity or indication of having been baked. The. dip of these beds 

 is N. 20° E. at an angle of 16°, and the tuff-beds overlying the lava have a 

 similar dip. Each pillow has a tachylytic selvage about 1 in. in thickness, 

 but the rock is noncrystalline at the centre. The pillows at the base of the 

 flow are somewhat irregular in shape, but higher in the mass they become 

 more spheroidal. One of the pillows near the base has a diameter of 30 ft. 

 Higher in the section they are smaller, decreasing in diameter, to about 

 2 ft., but towards the surface they again increase in size. Some of the 

 pillows are much elongated ; sometimes they show an indented peripherv ; 

 occasionally the indentation has penetrated to the centre of the mass, and 

 the upper half appears as if it had fallen over toward the lower half while 

 still in a viscid state. Vesicles are by no means prominent in the rock. 

 Occasionally large scattered ones occur, but on the tachylytic margin they 

 are small and rounded. At the bottom of the flow the vesicles are small, 

 and occur chiefly towards the exterior of the spheroid. Near the top of the 

 lava the pillows are almost free from vesicles. By infiltration of calcareous 

 solutions the rock in places becomes amygdaloidal. The fossiliferous lime- 

 stone which separates the pillows is indurated, but there is no indication of 

 alteration by heat. 



The broken-up pillow-lava, which is separated from the rock just 

 described by a thickness of 50 ft. of calcareous tuffs and interstratified 

 limestone, has always been referred to as an agglomerate or breccia ; but 

 it is clearly a pillow-lava that has been locally broken up during flow. 



The pillows vary much : some are similar to those already described, 

 others are much elongated and almost scoriaceous, the vesicles being- 

 abundant and much drawn out. The rock throughout is much more 

 vesicular than the lower lava. One pillow was noticed in which a large 

 central cavity was coated with tachylyte, as well as the periphery. There 

 is great variation in the size of the masses, the smallest having a diameter 



