Speight. — The Tertiary Beds uf the Trelissick Basin. 331 



Limestone, the " lower limestone " of Hutton and McKay, a some- 

 what flaky argillaceous stone, the lower part greenish, with small 

 fragments of volcanic ash ; thickness about 80 ft. 

 All these beds strike N. 15° W., and dip westerly at an angle of 15°. 

 When followed north towards Broken River the ash bed thins out and 

 disappears, the only indication of the presence of volcanic matter being 

 small particles scattered in places through the stone. The relations between 

 the upper and lower layers of the limestone are obscured between the two 

 rivers, but in the gorge of Broken River a clear-cut section shows no 

 decided line of demarcation between them, and they apparently grade into 

 one another insensibly. The spot, however, is somewhat difficult of access 

 except at times Avhen the river is quite low. 



From Home Creek the beds continue south across the Porter, but an 

 excellent section of the upper layer of limestone and the subjacent tuff 

 beds is seen just at the mouth of the Thomas River, the junction of the 

 two being quite conformable. Followed south, the limestones form the 

 highest point of the steep escarpments of Prebble Hill, facing east and 

 south, and the strike swings round as described in the section dealing with 

 the lower beds, till the upper limestone gorge of the Porter is reached. 

 (Plate XXI, fig. 2.) Here occurs the following sequence : — 



Limestone, full of coralline fragments like the upper layer in Home 

 Creek, 7 ft. thick. This bed can be traced round on the slopes 

 of Prebble Hill, so that it is continuous with the upper layer at 

 the mouth of the Thomas, but it has thinned out very much. 

 Volcanic tuff: Just as in Home Creek, the overlying limestone grades 

 down into this bed, but its lower portions are a true volcanic 

 tuff ; the total thickness is about 60 ft. 

 Limestone : The rock is flaky in its lower portion, but in the upper 

 parts it loses this character and takes on the nature of the so- 

 called upper limestone. Hutton says (loc. tit., p. 397), " On the 

 south side of Prebble Hill the limestone is divided into two 

 parts, the loAver of which is composed of comminuted fragments 

 of Bryozoa, Hydrocorallinae, &c, forming what is called a coral- 

 line limestone, thus differing altogether from its normal character, 

 and resembling the upper limestone presently to be described." 

 This statement is quoted as showing that lithological character will 

 not distinguish the two limestones, and that they are probably 

 one member of the series, and that they grade into one another. 

 At this point the beds strike almost due east, with a northerly dip of 50°. 

 From this locality the beds apparently run west, but they are hid by 

 the gravels of the plain till the north-east corner of Castle Hill is reached. 

 Hutton puts in a fault at this place with a downthrow to the north, whereas 

 McKay explains the arrangement by the presence of a fold. 



The mass of Castle Hill which dominates the interior of the basin, and 

 whose picturesquely eroded rocks are responsible for the popular name for 

 the locality (Plate XXI, fig. 1), presents a well-defined escarpment to 

 the east, but unfortunately it furnishes no clear-cut sections showing rela- 

 tions of the two limestones (Plate XXI, fig. 2). The exposure along the 

 eastern face of the hill shows that the beds have been folded into an anti- 

 cline and syncline on going from north to south, the anticline having been 

 eroded at the north-eastern corner of the hill (fig. 4). On a line about 

 300 yards to the west of this and parallel with it the hill has a dome 

 structure, and the syncline to the south apparently flattens out. In a 



