Vol. 55-] 



GEOLOGY OF ]\"OKTHEEN AIS'GLESEY. 



653 



scale in investigating the relations of the Green Series to the Ordo- 

 vician, and similar dislocations of the rocks are common in the 

 Northern Complex of Cemaes and Forth Wen. The older rocks here 

 crop up in strips between newer beds, and are bounded on one side 

 by a normal, on the other by a thrust-fault. I have detected a 

 number of each kind in the cliffs of the north coast, and others are 

 interred from the stratigraphy. The line of fracture of the reversed 

 faults as exhibited in coast-sections is often not straight, but a more 

 or less sigmoidal curve, and overfolds can be sometimes seen in 

 connexion with them. One such broken overfold is visible in bedded 

 quartzites, south of the graptolitic shale of Forth Fadrig, and 

 another example is seen in the ' quartz-knob ' at Llanbadrig Foint 

 (Trwyn y Buarth); see fig. 5. Of this knob Frof. Blake' says 





if 



Pig. 5. — Section^jn qvartz- 

 hnoh, Llanbadrig Foint. 



[Heavy lines denote shear-places.] 



1 = Quartzite. 



2 = Siaty beds with broken 



pieces of limestone ( = 

 crush- conglom erate ). 



cave 



* the phenomena here are those of ordinary faulting, resulting, not 

 from pressure, but from tension. We cannot get this quartz-knob 

 into its place by folding.' But an examination of the quartz-rock of 

 this small headland shows several polished planes of shear which curve 

 down into the sea and along which the waves have found lines of 

 weakness. Out of these curving shear-surfaces curve other surfaces 

 of shearing. The neighbouring rocks are crush-breccias and the 



1 Quart. Joum. Geol. Soc. vol. xhv (1888) p. 518. 



