402 Transactions. — Geology. 



thick, and is underlain by 10 feet of volcanic grit, below which 

 comes 40 feet of tufaceous greensands, much current-bedded, 

 and known as the " Fan coral-beds," from the occurrence of 

 Flabelluni laticostatum, Ten. -Woods. These rocks pass in an 

 easterly direction up Prebble Hill, but they are not connected 

 with those at the first gorge of the Porter. In Home Creek the 

 limestone is 60 or 70 feet thick, and the tufaceous greensands 

 50 feet, with a bed of blue clay, 1^ feet thick, between them and 

 the limestone. The dip in the Thomas River is 20° W. to 

 25° W., in the Home Creek about 20° W.S.W., and at the 

 natural tunnel through which Murderer's Creek joins Broken 

 Eiver the dip is 10° N.W. In Broken River the tufaceous beds 

 are thinner, but I did not examine them closely. At the 

 natural tunnel they have passed into a calcareous tuff, which I 

 did not recognize further north. An outlier of limestone occurs 

 on Flock Hill, and two inliers on the west margin of the basin : 

 one near the head of Moth Creek, the other a little north of 

 the White-water Creek ; but I did not examine them. 



Vulcanic Backs. — Scoria beds and tuffs are largely developed 

 in Coleridge and White-water Creeks. In Coleridge Creek some 

 of the scoria beds might almost be called agglomerates, and evi- 

 dently we are here near the orifice of a volcano which was in 

 eruption during the early part of the Oamaru period. Some of 

 the tuffs are tine-graiued, compact rocks of a blue-black colour, 

 and when broken present a sparkling crystalline surface, so that 

 they might, at first sight, be readily mistaken for lava streams. 

 But they eftervesce with acid, and when thin sUces are examined 

 with a microscope they are found to consist of fragments of a 

 deep brown-yellow palagonite, held together by a crystalline 

 calcareous cement ; some of the larger fragments contain 

 crystals of olivine, and occasionally there are separate olivine 

 fragments. Other tuffs are finer in grain, and effervesce very 

 slightly. The specific gravity of these tuffs varies from 2-10 to 

 2-70, according to the amount of calcitc they contain. In White- 

 water Creek, just above the Amuri limestone, there is a tacliy- 

 lyte lava stream. It is compact, black, dull, breaks up irre- 

 gularly under the hammer, and has a bluish tinge ou the surface 

 of the joints. Its specific gravity is 2-20. Under the micro- 

 scope, in very thin slices, it is seen to be a vesicular tacbylyte 

 of an olive-brown colour, studded with globulites arranged in 

 groups, either as clouds or as blackisli spots. 



In Home Creek, resting upon the Amuri limestone, another 

 similar palagonite tuff' occurs, compact, and of a blackish-green 

 colour. This bed is 8-10 feet thick ; the lower part is granular, 

 effervesces freely, and has a specific gravity of 2-10 ; the upper 

 part is finer in grain and effervesces sliglitly, its specific gravity 

 is 2-00. Under the microscope this tuff is seen to be composed 

 of angular fragments of brownish-yellow, or yellowish-green. 



