Volume 67 1958 > Volume 67, No. 2 > Blackstone, by I. W. Keyes, p 158-161
BLACKSTONE AS A descriptive term for a rock type frequently utilized in the fashioning of New Zealand stone adzes, is one that is widely used. Unfortunately, to many, this name will merely imply a rock of a distinct black appearance, and could include rocks like coal, obsidian and basalt. The rock generally referred to by the name blackstone, however, is an argillite of a specialized nature, the best quality of which is to be found in the Mineral Belt of the South Island of New Zealand.
The Mineral Belt is so named because of the great diversity of mineral deposits, igneous intrusions, and their consequent contact altered products. It extends nearly continuously from the upper waters of the Wairau River in the south, to D'Urville Island, some seventy miles to the north. It varies from half a mile to seven miles in width. The rocks of the Belt were formerly mapped as the Maitai Series 1 but are now included in the Te Anau Group, 2 and it is among them that the famous olivine rock from Dun Mountain—dunite, is found, as well as some of the serpentine rocks that were used for artifacts.
The Mineral Belt has often been referred to as one of the “red belts,” due to the rusty-red weathering of the ultrabasic rocks, and is characterized by the paucity of vegetation, due to the high percentage of magnesia contained in the soil.
Argillite, in its true form, is a hard, compacted, fine-grained sedimentary rock. Normally, it is too soft for the production of artifacts, and its laminated character would also render it unsuitable. However, at many places in the Mineral Belt, argillites have suffered alteration by contact metamorphism, brought about through contact with the intrusive bodies of ultrabasic rocks, and have been changed to hornfels. This metamorphic, or baking process, has almost completely fused the minute grains within the material, and produced an extremely hard and tough product, of an almost flint-like nature which is much harder than steel. The colour of this metamorphosed argillite or hornfels is usually jet black, but varies to a dark grey and a dull green, although several specimens have exhibited an almost milky grey colour. Occasional thin ribbon-like bands of dark quartz may be seen running through the material, especially in the lighter variety. The darker variety sometimes exhibits clearly defined shiny streaks, these perhaps, marking the presence of old shear lines. The great suitability of the metamorphosed argillite for artifacts, is proved by the numerous Maori quarry sites, and the vast assortment of adzes found, that are made from the material. Because of this fact, the rock has occasionally been loosely referred to as “axite,” by several New Zealand geologists- 159
Map of Eastern Nelson showing distribution of recorded quarry sites, and their geographic relationship to the basic and ultrabasic rocks of the area.
(personal communication). Tribes situated in the vicinity of Cook Strait made very wide use of this material, and artifacts of metamorphosed argillite have been found widely distributed throughout New Zealand. Many adzes exhibit a particularly fine polish, and it is material such as this that assisted in the development of a high standard of material culture, so often attributed to New Zealand's early inhabitants.
There are four previously recorded quarry sites for metamorphosed argillite within the Mineral Belt (see accompanying map). Skinner 3 records a site about nine miles from Nelson—the Rush Pool, where the exposed face of the argillite has been worked (? by fire), and hammer stones. Thomson 4 recorded a quarry on D'Urville Island, on a spur between Woodman's Bay, and the small bay nearer the French Pass. He mentions also the presence of a similar contact altered argillite of the Moehau Series, in a quarry ten miles north of Cabbage Bay, Coromandel Peninsula, where metamorphism was brought about by contact with a diorite intrusion. Although Thomson makes no mention of this quarry being of Maori origin, Duff 5 assumes it to be so. On the Lands and Survey one inch to one mile sheet, N39, for the area, a quarry is marked on the coast, which is undoubtedly of European origin, approximately ten miles north of Cabbage Bay. It is quite possible that the altered argillite, although perhaps similar to the Mineral Belt product, does not mark the existence of a Maori quarry site as thought by Duff, but is merely a misinterpretation of Thomson's none too clear statement. Duff 6 described two quarry sites at Whangamoa (Hebberd's and Oakley's), where outcrops have been worked also by utilization of large hammer stones. All these four above mentioned sites are notable for the large quantity and thickness of deposition of discarded flakes, showing that the material has been greatly favoured, and no doubt worked for a considerable period of time. The discovery of numerous carefully worked artifacts by Adkin in Horowhenua, 7 and Duff at the Wairau Bar, 8 belonging to New Zealand's earliest inhabitants, the ancient Waitaha, offer proof of the early discovery, and long utilization of this metamorphosed argillite as a suitable material for artifact fabrication.
Most use of the metamorphosed argillite appears to have been made on D'Urville Island, where quarry sites are particularly numerous. Here, as elsewhere (Dr. H. J. Harrington, Geological Survey, personal communication), this hard rock occurs as lenses, pockets, and large boulders in the serpentine, and also in places on the west edge of the Mineral Belt. All the lenses appear to have been worked, some considerably more than others, possibly in order to find the most suitable rock for utilization in artifact manufacture. Signs of quarrying activities are also noticeable inland from Squally Cove.- 161
Best 9 has described many of the adzes that he figured, as being composed of “black aphanite.” Aphanite (a rock term no longer in use), was a very general name for any dark, fine-grained basaltic rock, but many of the adzes illustrated have the appearance of being made not of basalt, but of blackstone. Many of the adzes exhibit, in their roughly-flaked form, the characteristic surface produced when such a flint-hard material is worked, and adzes of blackstone are certainly more numerous than those of basalt. Megascopically, the darker basalts can often be distinguished from the metamorphosed argillite, by the presence of phenocrysts of feldspar and pyroxene in the matrix. However, there are some fine-grained basalts without phenocrysts which would possibly require microscopic determination.
Because of the significance of metamorphosed argillite in connection with New Zealand archaeology, correct petrological terminology is of the utmost importance. Colour always plays an important part in the determination of minerals, but alone, is not sufficient to designate a distinct rock type. Greenstone, however, is the only exception to this, and the adopted practice, is to add in brackets the actual mineral, either nephrite or bowenite if it can be determined. So should be the case with blackstone. Because of its wide, and long standing use, the name blackstone would be difficult to discard, but due to the colour variations within the material, the name blackstone hardly seems applicable. If this name is still to be used, a short note of explanation should always follow (metamorphosed argillite), so as to distinguish this important material from all the other rocks of a similar black appearance, but of different mineralogical character. Better still, a more suitable determination would be—metamorphosed argillite, jet black or light grey, etc., in colour.
1 Bell, Clarke and Marshall 1911.
2 Wellman 1952:18.
3 Skinner 1914:324-329.
4 Thomson 1918:321-322.
5 Duff 1946:116.
6 Duff 1946:115-124.
7 Adkin 1948.
8 Duff 1950.
9 Best 1912.