Broadgate. — The ''Bed Eocks " and Associated Beds. 



81 



Under the microscope the grey chert shows quartz and feldspars as 

 in greywacke, and small amounts of the other minerals noted in that rock. 

 An analysis of a sample of grey chert, as free as possible from quartz 

 veining, is appended ; comparison with the analysis of a greywacke from 

 Breaker Bay shows how little the grey chert differs from greywacke. 



Argillite bands in the grey chert are generally distorted. The grey 

 cherts are readily altered by weathering, the effects of which are well seen 

 on the coast between Lyall Bay and Island Bay. It is these rocks which 

 A. McKay classed as the Otapiri series, and described as " gritty grey 

 sandstones decomposing to a light-brown colour " (8, p. 61). 



Table : Rock Analyses. 



(1.) Greywacke, Breaker Bay, Cook Strait. 



(2.) Grey chert (= altered greywacke), Red Rock Point, Cook Strait. 

 (3.) Argillite, Point Arthur, Wellington Harbour. 

 (4.) Green argillite. Red Rock Point, Cook Strait. 

 (5.) Red argillite. Red Rock Point, Cook Strait. 

 (6.) Diabase tufi. Red Rock Point, Cook Strait. 



(7.) Diabase, average analysis (from R. A. Daly, " Igneous Rocks and 

 their Origin "). 



(Analyses 1-6 by Dominion Laboratory, Wellington.) 



Red and Green Argillites. 



In the reports of the old Geological Survey the term " slates " has been 

 used for these rocks as also for those now called " argillites." The red 

 and green slates do not differ, except in colour, from the common argillites ; 

 they show parting parallel to the bedding-planes, and, save where weather- 

 ing has been active, slaty cleavage is no more developed than in the argil- 

 lites (9, p. 44 ; 18, p. 43). Their chemical similarity to a typical argillite 

 is seen by comparing analyses Nos. 3, 4, and 5. The name " argillite " 

 is here used for these rocks. The exposures of red argillites in the Wel- 

 lington Peninsula are indicated on the map (fig. 1). The outcrop at Red 

 Rock Point, illustrated by the section (fig. 2), is the clearest. Only in 

 the case of this outcrop is the development of green argillite in connection 

 with red argillite plain. Outcrops on hillsides are conspicuous by reason 



