184 Scientific Proceedings, Royal Dublin Society. 



All the bands are affected by incipient cleavage, the sericite and 

 streaks of earthy matter lying with their long axes athwart the 

 direction of the bands at an angle of about 40°, the long axes 

 of the quartz grains sharing the same direction. It is possible to 

 break a hand-specimen of the rock along the dirt bands, and the 

 fractured face exhibits a striation indicating sliding. Joint planes, 

 at right angles to the banding, are frequently undulated, showing 

 that the movement has not proceeded far enough to impose a 

 definite schistosity upon the rock. 



Having now shown that the quartzites under discussion were 

 originally good sandstones, and even more or less laminated sand- 

 stones, we may proceed to inquire into their abnormal and very 

 remarkable stratigraphical relations. 



Discordant stratigraphical position. — The curious isolation of 

 the quartz knobs described by Professor Blake, in Anglesey, and 

 the discordant strike of the quartzite of Brandy Hole, described 

 by Mr. Kelly, might naturally be expected to find their explana- 

 tion in movements subsequently produced to the deposition of the 

 stratigraphical system in which they occur : and a prolonged exa- 

 mination of the behaviour of the rock in the field leaves little 

 doubt on this point. The solution of the difficulty is to be found 

 in the differential movement of rocks of different degrees of 

 rigidity when subjected to powerful earth-pressures. 



In tracing well-defined beds of quartzite obviously interbedded 

 along the cliffs and coast of Howth, one is surprised to find them 

 suddenly and abruptly terminating in a joint plane, the joint 

 blocks which should have continued the direction of the bed 

 having disappeared and their place having been taken by the 

 slate of the district : or again, when examining the slate, one 

 sometimes unexpectedly encounters a huge parallelepiped of 

 quartzite, it may be six or seven feet in length and three or four 

 feet square at the base, lying athwart the general lie of the 

 country. This is evidently a joint block torn out from a bed of 

 quartzite and carried by the general flow of the country into a 

 novel position ; it is in fact a foundling produced by internal earth 

 drift. This movement of parts of beds out of their place makes 

 the surveying of the district a peculiarly difficult task, the more 

 especially in the northern part of the Hill of Howth, which consists 

 of the squeezed basal conglomerates of the Cambrian (?) system. 



