Igneous Rocks. 105 



the Causeway the cokimnar structure is remarkably developed 

 in the lower part of a flow, while the upper part, which has 

 cooled more rapidly, shows the characteristic "starchy" 

 structure. Spheroidal structure is common, and when 

 weather has attacked the surface it brings out the structure 

 known as "onion" structure. 



As the same structures are common to the Upper and 

 Lower basalts, and as there is no fixed lithological diff"erence 

 between the Upper and Lower sheets, it is impossible to 

 determine to which group an isolated specimen may belong: 

 it is only by tracing the field relationships of the rocks that 

 a determination can be relied upon. 



The various degrees of crystallization allow of distinguish- 

 ing the rocks into coarse dolerite or gabbro ; true basalt; 

 and on the margins of dykes or on the under surfaces of 

 flows, as at the Rathkenny mine, the glassy type (tachylite) 

 may be found. In the dolerite the olivene appears to have 

 consolidated first in the form of irregular grains ; plagioclase 

 felspar fiirms a meshwork in which augite has subsequently 

 crystallized in large ophitic masses. This order of crystalliza- 

 tion is, however, not constant. ^ 



Sir Archibald (ieikie has described the " segregation 

 veins" occurring in the dolerite of Portrush and Fair Head: 

 "These veins or seams differ from the rest of the rock 

 mainly in the much larger size and more definitely crystalline 

 form of their component minerals." - 



Basic Rocks, County Down. 

 Beyond the exposure of basic rock at Scrabo (ophitic 

 olivene dolerite), and a similar but smaller mass at Dun- 

 donald railway station, the exposures of basalt in County 

 Down are confined to the dykes, which in some districts are 

 very numerous and noteworthy. At St. John's Point, near 

 Ardglass, dykes occur along the shore, some of fine-grained 

 basalt, with large porphyritic felspars retaining their crystalline 

 form and structure unaltered. The two fine dykes, between 

 tides, just south of the Lighthouse, show this very clearly. 



1 Professor Judd on "Glomero-Porphyritic Structure," in Quart. Journal 

 Geol. Soc, xlii (1886). 



2 Volcanoes of Great Britain, vol. ii, page 300 <■/ .v^. 



