Igneous Rocks. 103 



crystallization, and marked from the others by the presence 

 of similar ochre and bole beds of much smaller dimensions. 



We find the first flows of the Lower basalt overlying the 

 Chalk or earlier rocks in all cases. The surface of these 

 earlier rocks have been denuded and weathered ; in many 

 places pockets in the earlier rocks contain flints and other 

 nodules which have been weathered out ; these flints are 

 much altered in colour ; the surface is coated with brown 

 ferruginous matters, and the body of the nodule has been 

 converted from the blue grey of the Chalk flints to reds 

 and browns through metamorphic action. In certain 

 places, as in the quarry at Whitehead, an old valley section 

 on the surface of the Chalk is exposed. On this land sur- 

 face the first outflow of basalt occurred, charring the scanty 

 covering of chalky soil that had disintegrated from the main 

 rock. The outlets for this lava material are to be found in 

 the numerous fissures in the Chalk and other rocks ; these 

 fissures are now filled with basic lava and widely known as 

 dykes. As well as the effusions from fissure eruptions, an 

 amount of material was derived from volcanic centres. The 

 necks of at least four such volcanoes are to be traced at 

 Carnmoney, Scawt Hill, Tiveragh, and Slemish, while at 

 Carrick-a-Raide, near Ballintoy, the agglomerate and tuffs 

 indicate a fifth eruptive centre. 



Excellent specimens of these minor flows are to be seen in 

 the many sections along the Antrim headlands. At the 

 Gobbins cliff, east of Island Magee, the various flows are seen 

 clearly differentiated from each other. Some of the sheets 

 are compact and massive black basalt ; others are vesicular 

 and amygdaloidal, often containing in their cavities zeolites 

 and other secondary products. The zeolites are frequently 

 drawn out in the direction of the flow, indicating a consider- 

 able degree of movement during the formation of the 

 vesicles. Other flows are traversed by a number of vertical 

 joints, and approach to rudely columnar basalt. The flows 

 differ in the degree of crystallization of the various minerals, 

 indicating different rates of cooling and different conditions 

 under which the magma cooled. Prequently one flow is 

 marked off from the next by a band of red ochre, the 

 remains of a soil weathered from the surface of the older 



