Speight. — Geology of the West Coast Sounds. 255 



Art. XXX. — Notes on the Geology of the West Coast Sounds. 

 By E. Speight, M.A., B.Sc, F.G.S. 



[Bead before the Philosophical Institute of Canterbury, 1st December, 1909.] 



The following notes are based on observations and collections made during 

 a fortnight's steamer-cruise in the Sounds in the summer of 1908-9. On that 

 occasion all the Sounds were visited, but the time was too short to do more 

 than collect specimens on the shore where landings were made. However, 

 some of these places have not been previously visited by a geologist. The 

 vast extent of country, the difficulty of travelling on its forest-clad and 

 precipitous slopes, and the small number of accessible outcrops of rock on 

 the shore-line of its deep, wall-sided fiords, render a satisfactory account 

 of its intricate geological structure absolutely impossible at present. It 

 furnishes perhaps the most complex of all the problems that New Zealand 

 geologists will have to solve. 



Sir James Hector (4, 5, 6) and Captain Hutton (8) have already given 

 a general description of the country, and Andrews (1, 2) has traced 

 out the evolution of its landscape in the light of modern physiography, 

 an account which has been generally indorsed by Marshall (14). Petro- 

 logical notes have been furnished by Hutton (9, 10, 11, 12), based on a few 

 specimens collected by himself at various times ; and more extended de- 

 scriptions have been made by Marshall (13, 14) as a result of collections 

 made by him during various excursions into the region. He has drawn 

 special attention to the magnesian rocks at Anita Bay (13), where he dis- 

 covered dunite, and harzburgite, and olivine-bearing marble whose origin 

 he ascribed to modifications in the neighbouring harzburgite, a conclusion 

 which he based on the evidence of a series of specimens showing transitions 

 between the two rocks. These rocks have been more recently referred to 

 by Finlayson in an interesting paper on the origin of nephrite, read before 

 the Geological Society (3). The present paper has been contributed in order 

 to supplement previous articles. The author has to express his great in- 

 debtedness to Dr. Marshall for the loan of a selection of microscopic sections 

 of rocks described by him in his papers. It has thus been possible to 

 identify some specimens here mentioned as being the same as described by 

 Dr. Marshall, or sUght variations from them. "^ - 1 A 



In order to give a general idea of the petrology of the area, each locality 

 visited will be taken in turn, and a short description will be given of the 

 rocks collected. It is impossible at present to give any account of their 

 field relations. The notes given are therefore unsatisfactory ; but the mere 

 indication that certain rocks occur in particular localities is considered 

 by the author to be of some importance, considering the extent of the area, 

 and the ignorance that at present exists regarding its geological structure. 



Before dealing with the petrology of the area, brief reference will be 

 made to one or two points of physiographical interest. The first of these 

 deals with certain shore-features. Even at the heads of the Sounds the 

 shoahng caused by the detritus poured in by large streams is limited to the 

 area close to their mouths. Extensive flats do occur, but they present 

 extremely steep faces to the sea beneath water-level. At the head of Wet 

 Jacket Arm, on sounding to get an anchorage on one of these flats, the 



