Igneous Rocks. 95 



north of Ballyniena. On Sandy Braes, near Tardree 

 Mountain, occurs the glassy variety of acid type known as 

 pitchstone. 



As well as the above localities, where the rocks are 

 developed on a large scale, there occur many smaller ex- 

 posures of acid intrusions, as in the rhyolitic intrusions in 

 the granite of Castlewellan. At Tornamoney, about two 

 miles north of Cushendun, there is a considerable exposure 

 of eurite. 



Granite. 



The granites of the Mourne mountains are easily acces- 

 sible from Belfast, and the district is one offering much 

 variety of scenery. For the various routes that may be 

 chosen in making excursions through the district, the Guide 

 published by the County Down Railway Company should 

 be referred to for reliable information and suggestions. 



The granite of the Mournes is an excellent example of an 

 acid intrusion or boss raising on its shoulders large masses 

 of Ordovician and Silurian rocks. These Silurian rocks 

 have subsequently been denuded from the higher peaks, 

 leaving the granite core exposed. In many places large 

 areas of the Silurian rocks are still left of consideral)le thick- 

 ness and at high elevations ; and in such rocks we find a 

 number of dykes which give us a suggestion as to the age 

 of the granite mass. Such Silurian remnants may be seen 

 on the top of Thomas Mountain on the flanks of Slieve 

 Donard, on the cap of Finlieve (1,888 feet high), and on 

 Slievemaganmore. These isolated patches are not the only 

 Silurian rocks to be found at considerable elevations, for 

 other Silurian rocks in situ may be found at even greater 

 elevations, as in the spurs dominating the Deer's Meadow. 

 Hence we may infer that the uplift affected a wide area, and 

 that these patches are the remnants not yet removed by 

 denudation. The intrusion of the granite has altered the 

 shales, baking and contorting them, so that small specimens 

 may often be found showing the crumplings of the strata 

 very clearly. 



In the patches above referred to, some of the dykes have 

 intruded before the uplift of the mass, so that the whole 



