Park. — The Loioer Mesozoic Bocks of Neiv Zealand. 381 



landing from Shaw Bay it bulges out into great lentil-shaped 

 masses resembling the " laccolites " of Gilbert. 



The actual line of contact between the porphyrite and 

 sandstone is clearly exposed in the road-cuttings, and, so far 

 as can be determined, the latter seems to have suffered very 

 little alteration, the only noticeable difference being a some- 

 what greater degree of induration than elsewhere. 



At the north head of Roaring Bay the dyke throws out 

 ramifying branches into the enclosing sandstone. 



The porphyrite is granitoid in texture, and weathers to a 

 soft crumbling yellowish earthy brown rock of a dark greyish- 

 green or blue colour, in which the large well-developed 

 phenocrysts of plagioclase become conspicuous when parti- 

 ally kaolinised. It is tough, but not hard, and ought to form 

 a building-stone of superior quality. 



In polarised light, the rock is seen to be composed prin- 

 cipally of plagioclase, almost to the exclusion of the augite. 

 The latter surrounds the feldspars like a cementing medium, 

 filling the interspaces and consequently occurring mostly as 

 narrow irregular aggregates. 



The plagioclase is generally kaolinised, opaque, and turbid. 

 The augite is mostly altered to greenish serpeutinous matter 

 and secondary magnetite. 



At the point where the road leading to the lighthouse 

 leaves the beach, near Boat-landing, there is a bed of clay- 

 stone a few feet below the dyke which contains a considerable 

 number of marine forms m a fair state of preservation, 

 occurring principally as casts. Aiiiong the genera identified 

 here were Spiriferma {Psioidea of Hector), Spiriferina (two 

 sp.), Epithyris, Bhynchonella, and Pleurotomaria. Besides 

 these, fragments of a bivalve shell resembling Modiolopbis 

 were not uncommon. 



This fossiliferous horizon is of especial value, as its con- 

 tents establish the Triassic or Permo-triassic age of the for- 

 mation, and enable the position of the associated beds to be 

 correlated with the Nelson and Southland Lower Trias or 

 Permian beds. 



In the sandstones and underlying the Spiriferina beds 

 there is a thin stratum of indurated sandstone crowded in a 

 few places with casts of single valves of a thin bivalve shell. 

 On the surface of rounded boulders, when worn at right 

 angles to the plane of deposition, the casts are seen in section. 

 The profile of the single valves gives a concave appearance 

 somewhat resembling the ventral valve of a Productus, and 

 the resemblance to that genus becomes more deceptive when 

 two valves happen to fit into each other. 



The casts are those of a very thin gibbose conchiferous 

 shell from 0-5 in. to 0-75 in. high and about the same width. 



