74 E. D. Isaacson — GraptoUte Rochs of New Zealand. 



VI. — Notes on the Geaptolite-beaeing Rocks oe New Zealand.^ 

 By E. Douglass Isaacson, F.G.S. 



THE series of beds containing graptolites in New Zealand occur in 

 the Whakamarama district, which is situated in almost the 

 extreme north-west of the South Island. They consist of inter- 

 calated bands of quartzite and carbonaceous argillites, with a north 

 and south strike and dipping at a low angle to the west. As a result 

 of the natural erosion of the land surface taking place more rapidly 

 in the slaty layers than in the harder quartzites, the ridges and stream 

 valleys exhibit a noticeable parallelism, those streams which enter 

 the sea on the western coast usually taking a very sharp bend to the 

 west, and with a somewhat gorgy channel to the sea. The valleys 

 are for the most part densely clothed with forest trees, while the 

 ridges of quartzite are barren, with the exception of a stunted 

 growth of manuka {Leptosperrmim scopariiim and L. ericoides), and in 

 places a covering of peat to a depth of a few inches. 



The deposition of the sediments has been characterized by some- 

 what sharp changes, due no doubt to the alternate elevations and 

 depressions of the ocean bed, which appear to have succeeded each 

 other somewhat rapidly about this period. Neither the beds of 

 quartzite nor the argillites are of any great thickness, and have an 

 approximately equal development. 



In the argillites fossils have hitherto been found only in the neigh- 

 bourhood of the auriferous reefs of the Golden Ridge, Whakamarama, 

 and are restricted to a band of carbonaceous argillite which con- 

 stitutes the walls of the reef in this locality. As at Bendigo, Victoria, 

 the graptolite beds are in intimate association with the auriferous 

 reefs, and, moreover, the prospecting for fresh reefs in this locality 

 seems to point to the fact that their intimate association is necessary 

 for a payable reef to be discovered. The argillite bed is some 

 hundred feet thick, and includes a band more highly graphitic than 

 the remainder from which the fossils are obtained. Owing to 

 the inaccessibility of the country and the absence of outcrops, the 

 fossiliferous beds have not yet been by any means fully investigated, 

 and it is highly probable that further work may yet lead ta 

 important discoveries. The argillites have a somewhat schistose 

 appearance from the parallel plates of graphite, the other component 

 being fine silica in a cement medium. 



Though not hitherto discovered m situ, an interesting rock-specimen 

 obtained in this locality by the author was a spotted slate, which 

 under the microscope was observed to contain excellent crystals of 

 chiastolite with carbon inclusions arranged in a cruciform manner. 

 This rock probably was a result of thermal metamorphism, caused by 

 the intrusion of a granite boss to the southward, and is a rare 

 instance of this change in the country. The quartzites, variously 

 also termed chert and grauwacke by different authors, consist 

 almost entirely of a mosaic of subangular quartz- grains. As 

 mentioned above, the graptolites have up to the present only been 



^ See paper by Mrs. E. M. R. Shakespear, D.Sc, "On some New Zealand 

 Graptolites," Geol. Mag., Dec. V, Vol. Y, April, 1908, pp. 145-8. 



