94 WALKS AND TALKS. 



later Tertiary, ages as signalized in the history of the world 

 by outflows of molten lava primarily from fissures, but sec- 

 ondarily building up small and moderate sized cones in great 

 abundance, and not a few stupendous mountain piles reaching 

 to eighteen thousand feet. 



In remote geologic ages lava eruptions were of frequent 

 occurrence but less frequent and less copious than in later 

 ages. During 'the Triassic Age (see Table, p. 73) many erup- 

 tions of lava occurred, both in Europe and America. The 

 Palisades of the Hudson; the cliffs of Meriden, Connecticut, 

 and East and West Rock, New Haven, are ancient lavas of 

 this age. Much farther back in geological history, in Cambrian 

 time, or as some think before Cambrian time, vast and repeated 

 outflows of lava took place which remain to-day uplifted in 

 Keweenaw Point and He Royale. The native copper is 

 found imbedded in these ancient lavas. 



A fissure filled with rock-material solidified from a state of 

 fusion, is a dyke. Sometimes the formation containing the 

 dyke is more friable than the lava, and weathers away more 

 rapidly. The dyke then projects above the surface like a 

 vertical wall. Certain varieties of lava called basalt possess 

 the peculiar property of assuming a columnar structure while 

 cooling. The longer axes of the columns are ranged at right 

 angles with the cooling surfaces. Thus, when the basalt cools 

 in a fissure, the columns lie transversely from wall to wall. In 

 most cases, the columns are vertical. This is thought some- 

 times to result from cooling under the sea ; but probably when 

 a sheet of basalt rests on the surface of the earth, the atmos- 

 phere above and the earth below are cooling surfaces of the 

 requisite efficiency to develop vertical columnar structure. 

 The columnar structure induced to an imperfect extent, in 

 basaltic rocks of He Royale may be conceived as produced in 

 the bottom of the sea; but the columnar structure in the 

 canon of the Columbia must have been acquired upon the 

 land. The columns, in some cases, rest with their ends 

 directly on a bed of pebbles and sand not over a hundred 

 <eet thick, and bearing the evidences of torrential action 



