TWO NEWLY-DISCOVERED WHIN-DYKES 333 



The higher portion sends out several small lateral offshoots to 

 the south and a long one to the north. This is intruded in a 

 coal seam, with thin shaly partings, and the coal is baked to 

 a cindery mass having the prismatic structure characteristic 

 of many cinder-coals. The prisms tend to set normally to the 

 whin surfaces, of which there are two at right angles to each 

 other. At the upper exposure the dyke, now increased in 

 breadth to 12 inches, is again displaced to the north, the 

 break occurring along a shaly parting in the sandstone, and 

 the two portions being connected by a band of whin about 

 4 inches broad. 



The general appearance of the dyke and cinder-coal is 

 shewn in Fig. 1. The phenomena exhibited by the dyke 

 would be explicable as the results of intrusion up a highly 

 jointed rock, the molten whin breaking across from one joint 

 to another and the whin-filled fissure closing up, before 

 solidification, at its eastern end. It may be noted, in 

 amplification, that the slope of the cliff virtually gives three 

 vertical sections of the dyke at the positions where displace- 

 ments occur. 



The direction of the dyke is 20 N. of W., the same as the 

 Collywell Dyke and the master-joints and small fissures in the 

 Crag Point sandstone; its hade is about 15 S. In the natural 

 section, the hade appears much greater owing to the slope of 

 the cliff and the fact that the dyke meets it at an acute angle. 

 Hence Fig. 2, which shows the position of the Hartley and 

 Collywell Dykes in section, is drawn at a slight angle to the 

 cliff-line and at right angles to the direction of the two dykes. 

 It will be seen from the sketch that both intrusions occur in a 

 mass of shales and sandstones, which are broken up by a 

 number of small, normal faults. 



To amplify the relations between the dykes, a brief account 

 of the outcrops of the Collywell Dyke on the foreshore is 

 necessary. At its most easterly exposure, off Crag Point, the 

 dyke is intruded at the northern edge of a fissure about 20 feet 

 wide. This fissure is traceable across Collywell Bay to the 



