476 Transactions. — Geology. 



and my own examination of the peninsula breccias, I cannot 

 but think that, though these are doubtless outcrops of a vol- 

 canic bed which owed its origin to Waitakerei Oligocene 

 vents, those vents w r ere not identical with that which pro- 

 duced the Cheltenham breccia. Probably it was equally near 

 (for the lava fragments are quite as coarse), but from the 

 thinness of the bed one is led to infer that the vents which 

 formed it were smaller, or that they continued active for a 

 comparatively short period. 



The Parnell Grit. 



The Parnell grit, like the Cheltenham breccia, presents a 

 somewhat bedded appearance, due to the linear arrangement 

 of fragments of similar size. In this case, however, the bands 

 are much more regular, and the coarsest fragments — the size, 

 perhaps, of marbles — are all at the bottom, from which point 

 the bed grows gradually finer till it shades off into a sand- 

 stone. The outcrops of the Parnell grit everywhere present 

 the most beautiful examples of the shading-off of one rock 

 into another. The red rounded lapilli of the lowest layers 

 are gradually replaced by smaller ones, and these by yet 

 smaller, till at length they can no longer be observed with the 

 naked eye, and on a fresh fracture the rock has all the appear- 

 ance of a blue sandstone ; nor is it possible to say where the 

 sandstone begins and the breccia ends. The lapilli are very 

 uniform in size in each layer, which seems to indicate a 

 distant origin. The average thickness of the bed is about 

 18 ft. Fossils do not seem to be as abundant as in the older 

 beds. 



The weathering is extremelv characteristic and useful. In 

 the coarser parts it is similar to that of the Cheltenham 

 breccia, but in the upper parts concretionary or spheroidal 

 structure is usually developed. Round the shells of brown 

 iron-oxide the dark colour which the bed usually presents is 

 absent, so that the concentric layers are plainly visible. This 

 type of weathering seems to be almost confined to this bed, 

 none of the Waitemata sandstones exhibiting it ; and it is 

 invaluable as a means of detecting the presence of the bed in 

 inland outcrops, where the colour has generally all been 

 leached out and the bed is left an (apparently) white crumbly 

 sandstone. Under sea-water neither this bed nor the Chel- 

 tenham breccia weathers more than a few inches, and the 

 feldspars remain comparatively fresh. 



As at Cheltenham, a very common feature of the bed is the 

 occurrence of zeolite " dykes," dividing the surface of the grit 

 into irregular polygonal plates. These dykes, when split, fre- 

 quently show beautiful dendritic manganese markings. The 

 markings closely resemble those formed by moist emery- 



