334 DR - J- A - SMYTHE ON 



cliff just north of the well and is represented by C in Fig. 2. 

 Throughout its length it is filled with a breccia of thick sand- 

 stones and shales, dipping vertically or at a high angle north- 

 wards. As it passes, owing to the general north-westerly dip 

 of the rocks, from the sandstones of Crag Point to the shales 

 above, which occupy the bay, the fissure narrows considerably, 

 and in the cliff-section is only 5 feet wide. The Collywell 

 Dyke is intruded in this fissure for some distance, but it 

 changes position gradually when traced inshore from the 

 northern to the southern edge. The latter position is attained 

 when the fissure has reached the shales. In its further 

 passage up the fissure (this is, inshore) the dyke becomes 

 broken up and sends out several small apophyses to the south. 

 In favourable circumstances, a few isolated exposures in the 

 sandy bay intervening between the rocks and the cliff-section 

 show that it has left the fissure, and in the section itself it is 

 found 40 feet south of it. 



Now there is no trace of the Hartley Dyke on the foreshore, 

 although in many places there is a continuous exposure of 

 rock, and unless the direction of the dyke has changed very 

 suddenly it should be visible, as the Collywell Dyke is. 

 Considering this, along with the tendency of the Collywell 

 Dyke to divide into branches, as seen in the cliff-section as 

 well as along its outcrop, and to send out strings on its south 

 side, it appears probable that the Hartley Dyke is an offshoot 

 of the Collywell Dyke. If this be so, then it is interesting to 

 note that the intrusive energy of the offshoot is greater than 

 that of the parent mass, for it has pierced the strata which 

 have effectively checked the upward motion of the latter. 



2. The Basalt. The lower exposure of the dyke consists 

 of a soft, whitish rock, jointed horizontally and bearing little 

 or no resemblance in the field to ordinary basalt. This rock 

 contains 13*66 per cent, of carbon dioxide, an amount equiva- 

 lent to 31 per cent, of calcium carbonate. The upper exposure 

 is more whin-like in appearance, the rock being rusty in 

 colour and weathered into roundish blocks which show the 

 typical exfoliating structure. When broken open, these 



