Speight. — Geologn of the Trtlissick Basin. 157 



Art. XIX. — Further Notes on the Geology of the Trelissick or Castle Hill 



Basin. 



By E. Speight, M.Sc, Curator of the Canterbury Museum. 



[Bead before the Philosojihical Institute of Canterbury, 18th September, 1918 ; received by 

 Editor, 30th December, 1918 ; issued separately, 26th May, 1919.'] 



The following additional notes with regard to the Trelissick Basin are made 

 as the result of further examination since my former paper* was published. 

 The first matter concerns the existence of another detached block of lime- 

 stone on the western side of the area at the base of the Craigieburn Range. 

 This was hidden by bush during my former examination of the locality, 

 but fire has swept over it and exposed the rock clearly to view. I should 

 have observed it, however, had I ascended the Hog's Back Creek about a 

 quarter of a mile above the turning it makes on striking the Hog's Back 

 Ridge, as the bed crosses the creek and is exposed on its banks under the 

 covering of shingle. The limestone here forms a heav}^ baud, striking 

 east-north-east, pointing towards the saddle between the Hog's Back Creek 

 and Waterfall Creek in one direction, and iu the other runnmg towards one 

 of the spurs of the Craigieburn Range. In the creek the bed is 100 ft. 

 thick, dipping generally north-north-west at an angle of 85°, but is much 

 contorted in places. It is well jointed, slightly crystalline, and distinctly 

 coralline. When traced towards the south-west through the burnt bush the 

 dip flattens out and has a general direction of south-south-east at an angle 

 of 45°. The relations to the greywackes at the point where it abuts on 

 the spur of the Craigieburn Range are obscured by bush and a covering of 

 loose debris, but the general circumstances suggest a fault contact, and the 

 relations to the limestone of the Hog's Back Ridge are also difficult to 

 make out, but some strong deformation, either of faulting or acute folding, 

 certainly occurs between them. 



Another detached block of limestone occurs on the north side of the 

 Long Spur, which divides the basin of the Hog's Back Creek from that of 

 the Thomas River. This dips west at an angle of 45°, but at its southern 

 end the strike turns round so that it becomes north-east. The rock is much 

 brecciated. It is possible that this forms a syncline with the block of rock 

 just referred to, the uppermost members of the Tertiary sequence being 

 developed in a basin shape between them. 



Near the southern end of the Hog's Back a fossil locality was discovered 

 in the bed of the creek. The bed is 3 ft. in thickness where ex]:>osed, and 

 consists of sand and shell layers, striking east, and dipping south at an 

 angle of 75° — that is, parallel to the limestone of the Hog's Back in its 

 vicinity. The following fossils were obtained : Cucullaea ponderosa Hutt., 

 Fulgoraria arabica elongata (Swains.), Paphia cvrta (Hutt.) 



My attention has been drawn by Dr. W. P. Evans and Mr. A. E. Flower, 

 who were recently camping in the district, to another important fossil 

 horizon in the Thomas River. This occurs in the bed and on the north 

 bank of the stream just op2:)osite the" junction of the little creek that flows 

 past the farm and joins the Thomas in the vicinity of the old wool-scouring 

 works. The strata consist here of sands, sandy clays, and dark sandy shales 

 with numerous specimens of Ostrea ingens. Underneath the oyster-bed are 



* R. Speight, The Stratigraphy of the Tertiary Beds of the Trelissick or Castle 

 Hill Basin, Trans. N.Z. Inst., vol. 49, p. 321, 1917. 



