IRISH TERTIARY RHYOLITE. 



315 



Basalts of North of Ireland. The coast between Belfast Lough 

 and Lough Foyle is one boundary of a large tract reaching westward 

 to Lough Neagh, and including the river Ban, which is almost wholly 

 occupied on the surface by basaltic rocks, rising at intervals to 

 eminences of 1320, 1820, 1864 feet above the sea. Under this im- 

 mense overlying mass of basalt are found several members of the 

 secondary series of strata not known elsewhere in Ireland, i. Chalk, 

 agreeing with the lower beds of the English series. 2. Mullattoe, 

 an Irish name for the Hibernian Greensand of geologists. 3. Lias 

 limestone (without any other rock of the oolitic system). 4. Beds 

 of red marl, gypsum, and salt, resting on variegated sandstone. 5. At 

 the north-eastern and south-eastern extremity, coal-measures, con- 

 sisting of red sandstones and shales with inferior coal, appear below 

 all the other strata. The mulattoe and lias are often wanting in the 

 section. The superincumbent basalt is estimated to have an average 

 thickness of 545 feet (in Benyavenagh it is 900 feet, in Knochlead 

 980 feet), and its superficial extent 800 square miles. 1 



The immense mass of doleritic rocks in this district exhibits, 

 besides basalt, which' is the most abundant material, several other 

 varieties of rock. Near the Causeway, the cliffs consist of alter- 

 nating basalt and red ochre, in the following order downwards : 



1. Basalt rudely columnar, 60 feet. 



2. Red ochre or bole, 9 feet. 



3. Basalt irregularly prismatic, 60 feet. 



4. Columnar basalt, 7 feet. 



5. Intermediate between bole and basalt, 8 feet. 



6. Coarsely columnar basalt, 10 feet. 



7. Columnar basalt, the upper range of pillars at Bengore Head, 54 feet. 



8. Irregularly prismatic basalt, 54 feet. In this bed the wacke and wood coal 



of Port Noffer are situated. 



9. Columnar basalt, forming the Causeway by its intersection with the plane 



of the sea, 44 feet. 



10. Bole or red ochre, 22 feet. 



11, 12, 13. Tabular basalt, divided by thin seams of bole, 80 feet. 

 14, 15, 1 6. Tabular basalt, occasionally containing zeolites, 80 feet. 



1 See also Lee's Note-Book of a Geologist. 



