222 



STRUCTURAL GEOLOGY. 



Giant's Causeway in Ireland, and FingaFs Cave on the 

 Island of Staffa, are good examples. The Giant's Cause- 

 way is a sea-cliff of columnar basalt, consisting of many 

 layers, with softer material between, and the whole rest- 

 ing on stratified rock. By the action of the sea and air 

 the separated and disjointed columns are undermined and 

 fall to the base of the cliff. In this country, the Pali- 

 sades of the Hudson River, and Mounts Tom and Hoi- 

 yoke in the Connecticut River Valley, are good examples. 

 Fine examples are found also in the trap of Lake Supe- 

 rior (Fig. 132). But the finest in this country are the 



FIG. 132. Basaltic columns on sedimentary rock, Lake Superior. (After Owen.) 



basaltic cliffs of Columbia and Des Chutes Rivers in Ore- 

 gon. On the Des Chutes River at least thirty lava-layers 

 may be counted, one above another, each entirely com- 

 posed of vertical columns. 



Volcanic Conglomerate and Breccia. If a lava- 

 stream runs down a stream-bed or a shingly beach, it 

 gathers up the pebbles and forms with them a conglome- 

 rate differing from aqueous conglomerate in the fact that 

 the uniting paste is igneous instead of sedimentary. So, 

 also, a lava-stream may gather up rubble and form a 

 volcanic breccia differing in the same way from sedimen- 

 tary breccia. 



