57 



-formation. Lithologically, they are a quartz-felspar porphyry, 

 consisting of a felspathic matrix, enclosing well-developed 

 crystals of quartz and orthoclase. From the presence of these, 

 the rocks have a characteristic speckled appearance, which is 

 very marked on the surface of a fresh fracture. Their colour is 

 various, the matrix and the porphyritic felspar ranging from 

 white to a reddish-brown; the quartz is usually black when 

 looked at in the mass of the rock, but in a microscopic slide it 

 appears beautifully transparent, and polarizes with the most vivid 

 hues. Orthoclase is the only felspar present, no plagioclase 

 having been found in any slice prepared. In structure the rock 

 is eminently prismatic. This is very plainly shown in several 

 sections, but especially at the spot marked " c^" on the map, 

 where a series of nearly perpendicular, though short, columns 

 extends right across the stream. At the margin of the bank, on 

 either side, the porphyry is overlain by about 10 or 12 feet of 

 the polyzoal rock, the junction of the two formations being very 

 clearly marked, on account of their totally different character. 

 The outline of the country in this place is irregular, and I am 

 inclined to think that there must have been a trough or narrow 

 depression in the porphyry before the deposition of the tertiary 

 strata upon it. Resting upon the polyzoal rock is the basalt, and 

 the stream, having cut its way through the two uppermost forma- 

 tions, has exposed the underlying trap, wdiich now forms the bed 

 of the creek. Higher up the stream, the upper beds lie imme- 

 diately upon it, and fragments of the rock are occasionally un- 

 earthed when digging for fossils. These porphyries are unknown 

 anywhere to the south of the Grange Burn, but they can be traced 

 in a north-easterly direction from it for fully 30 miles. The 

 width of the outcrop varies, but in the neighbourhood of the ter- 

 tiaries the rock is seen only in the Grange Burn, there being no 

 sign of it in any part of Muddy Creek, neither in situ nor as 

 fragments among the fossil deposits. It is unlikely, therefore, 

 that the porphyry would be met ^vith in sinking below the bed 

 of the last-named stream, and the question as to what the under- 

 lying formation really is has yet to be solved. 



The presence of the porphyry aids in determining the thickness 

 of the upper beds. As the Grange Burn is foUowed upwards, a 

 point is reached where this rock just ceases to be covered by the 

 tertiaries, the basalt instead resting immediately upon it. The 

 ■elevation of this spot is 450 feet above sea level, and, therefore, 

 40 feet higher than the nodule band at the base of the upper 

 zone. 



^^iTo estimate can be formed as to the thickness of the remaining 

 portions of the tertiary series, viz., the lower beds of Muddy 

 Creek, the polyzoal rock, and the Portland beds. Near the top 



