332 DR. J. A. SMYTHE ON 



position. The augite which occurs in granular patches and, 

 abundantly, in skeleton crystals, is almost completely replaced 

 by calcite. Irregular green patches, possibly of devitrified 

 and decomposed glassy matter, are fairly common, as are also 

 amygdaloids, filled with calcite, and surrounded tangentially 

 by felspar laths. Calcite also occurs in irregular masses 

 sometimes penetrated by shafts of felspar, the ends of which 

 are weathered. Some of the sections contain porphyritic 

 felspars, usually single, occasionally in small groups. These 

 are strongly zoned, chemically, when in contact with the 

 basalt, and are traversed by irregular cracks filled with 

 calcite ; in all respects they are similar to the anorthite 

 phenocrysts of the Tynemouth Dyke. The rock is cut by 

 thin veins of calcite which sometimes enclose fragments of 

 whin. In one case, such a vein breaks through a porphyritic 

 felspar crystal, faulting it slightly without disturbing the 

 optical continuity of the separated portions. 



Petrologically, the rock resembles that of the Tynemouth 

 Dyke, and some of the observations seem to point to slight 

 movement in the dyke «after consolidation. 



The Hartley Dyke. 



i. Field Relationships, This dyke cuts through the thick 

 shales and thin sandstones which overlie the massive grits of 

 Crag Point. There are two exposures in the cliff-section, the 

 lower in black and sandy shales, the upper in the reddish 

 sandstones which, a few yards north, are overlain by the 

 yellow sands of Permian Age. The vertical distance between 

 the two exposures is about 20 feet, the dyke being effectively 

 concealed in this interval by thick soil and vegetation. 



The dyke, where it emerges from the talus at the foot of the 

 cliff, is 6 inches wide and, like its neighbour the Collywell 

 Dyke, appears to come to a head. It is, however, only 

 shifted, for the running is taken up a little to the north, and 

 the dyke then rises in sinuous fashion for 3 feet, when another 

 break with a displacement of 6 inches to the north occurs, 

 the two portions being connected by a very thin string of trap. 



