178 Scientific Proceedings, Royal Dublin Society. 



occurs, with tlie bands at right angles to the direction of the 

 cracks. In this rock a good deal of magnetite is disseminated in 

 small crystals, in one instance a crystal was observed inclosing a 

 small grain of quartz ; it would appear, therefore, to have been 

 formed in place. There are not so many signs of crushing in this 

 rock as I expected to find, but traversing one slice is a zone of 

 granulitic quartz which I take to indicate a line of yielding and 

 consequent crush. One or two grains of decomposed felspar were 

 noticed. 



The grains lie with their largest faces looking in one direction, 

 so that in section the long axes of the approximately elliptical 

 outlines will be found running parallel to each other. In con- 

 nexion with the banded extinction is the anomalous axial figure 

 given by many of the grains, which is evidently that of a biaxial 

 mineral, from which we may conclude that the pressure to which 

 the quartz has been exposed has thrown it into a state of per- 

 manent strain.^ 



Red Roch, Howth. — In addition to white, gray, and yellowish 

 quartzite, red or liver-coloured beds are met with as at Eed Rock 

 and other parts of Howth. The presence of red ferric hydrates, 

 earthy matter, and a little more sericite than occurs in the pre- 

 viously described quartzites, renders the grains of quartz (which 

 still constitute by far the larger part of the rock) very sharply 

 defined ; many are remarkably round, so that the thin slice under 

 the microscope sometimes reminds one of the polished face of a 

 piece of Herefordshire pudding-stone. The ferruginous earthy 

 matter and the sericite are drawn out along lines of flow which bend 

 round the quartz grains like stream lines round a boat (fig. 3), just 

 as one also sees around the eyes of a crushed igneous rock, or in 

 garnet schists where the mica fiows round the fractured garnet 

 crystals. Were the Old Red Sandstone conglomerate to become 

 converted into quartzite by pressure, as no doubt in many places 

 it has, it might be expected to produce on a grand scale just such 

 a rock as this is on the minute. 



Signs of crushing do not seem to be quite so common as in 



1 [Note added in the Press. — I have since ascertained that a good deal of this 

 rock consists of large quartzite pebbles, some as mucli as six inches in diameter. The 

 purity of the rock has led to its conglomeritic nature being overlooked.] 



