Igneous Rocks. 93 



even gold ornaments liave been unearthed. Many of these 

 objects, alas ! find a final resting-place, like Irish exiles, far 

 from home, instead of in well-organized local museums. 



KiNEOUS ROCKS. 



THE province of Ulster is extremely rich in the variety 

 of igneous rocks which it contains; our counties of 

 Antrim and Down presenting many problems worthy 

 of the petrologist's attention. In Antrim extensive lava flows 

 of Tertiary age cover most of the plateau. The outpourings 

 of this period are not confined to the basic types alone ; 

 the acid type of igneous rock is also represented by the 

 rhyolites of Tardree and certain districts north of Eallymena. 

 In the granites of Down we have a rock that is of consider- 

 able economic importance : a portion of this granite is of 

 undoubted Tertiary age, while another portion is certainly 

 of much greater antiquity. The dykes and sills of the Ards 

 peninsula also contain many Intermediate type of rocks, 

 such as lamprophyres or mica-traps ; while in the andesites 

 of the coast south of Newcastle we have the Intermediate 

 igneous rocks with plagioclastic felspars. There are con- 

 siderable difficulties encountered in attempting to deal with 

 the igneous rocks in their historical sequence, as in many 

 localities the age of the rocks cannot be determined except 

 within very wide limits : for instance, over the wide Silurian 

 uplands of Down we constantly meet with igneous intrusions 

 which we may broadly say are post-Silurian, but in many 

 places in this county where these igneous rocks occur we 

 have no representatives of the sedimentary type of later age 

 than the Silurian by which we can delimit the age of the 

 igneous rocks in question : therefore we propose to deal 

 with the rocks according to a petrological classification. 



Igneous rocks may first be roughly classified, according 

 to the percentage of silica which they contain, into Acid 

 Rocks, Intermediate Rocks, and Basic Rocks. These three 

 great divisions may be further qualified, according to the 

 degree of crystallization which the rocks present, into holo- 

 crystalline, microcrystalline, and glassy groups. These 

 groups in turn may present varieties in composition or in 

 the degree of crystallization which they present. 



