I02 Guide to Belfast. 



about four feet wide, and rises through the schists. Being 

 of a dark brown colour, it is readily distinguished from the 

 containing rock. In it are found crystalline hornblende 

 and biotiie, in an altered felspathic base. 



BASIC ROCKS. 



The chief development of this type is to be found widely 

 distributed over the county of Antrim in massive sheets ; 

 the same type is also found occurring as dykes and sills in 

 both Antrim and Down. Perhaps the best locality for the 

 study of the mode of occurrence is to be found at Scrabo 

 Hill, near Newtownards ; here the hill has been dissected 

 out in quarrying operations to such an extent that the 

 connection between the dykes and sills and the over- 

 lying basalt is very apparent. Basic dykes have been 

 referred to as cutting through the granite of the Mourne 

 mountains. 



The Basaltic Lava Flows may be divided into Lower 

 and Upper series. Between the two series occur those 

 beds of iron ore and bauxite that are of considerable 

 economic importance. In many places this interbasalt 

 series is of interest to the palaeobotanist as containing leaf 

 beds, as at Ballypallidy, or lignites, as at Ballintoy and 

 other places : it is from the character of the flora found here 

 that the age of the basalts may be determined, as well as 

 by correlating the series with the basalt flows of the western 

 islands of Scotland. The thickness of these interbasalt beds 

 in many places, the nature of the material of which they are 

 composed, the presence of rolled and water-worn pebbles 

 of lower basalt, and the widespread occurrence of rhyolite 

 far removed from any known extrusions of this rock, indicate 

 the fact that a considerable interval of time elapsed between 

 the flows constituting the Upper and Lower sheets. In this 

 interval vegetation had established a footing, and in the old 

 valleys formed on the surface of the Lower basalt sheet, 

 lakes and marshes provided a collecting-ground for the plant 

 remains referred to. 



The two main divisions or sets of basalt flows have them- 

 selves been built up of similar minor flows, each differing 

 from the succeeding or the earlier one in texture and 



