78 



BRITISH EOCENE ELORA. 



Oregon another enormous tract, estimated to equal the whole united areas of Erance and 

 England, has been overwhelmed, and become an undulating plain of Basalt at so recent 

 a date that it has been thought likely that the latest flows may have taken place within 

 the human period. All these are believed to have welled up from great fissures, for no 

 conceivable number of cones or craters would be adequate to produce such vast, level, 

 • and continuous sheets. They are termed "massive eruptions," and "represent the 

 grand fundamental character of vulcanism, modern volcanic cones being regarded as 

 • merely parasitic excrescences on the subterranean lava reservoirs, very much in the 



relation of minor cinder cones to their parent volcanoes."^ The distinguishing feature 

 of lavas erupted in this way is their horizontal bedding and the rarity and massive size 

 of the intersecting dykes, contrasted with those which are found diverging in all 

 directions from true craters that have been denuded of ash. 



The Basalts of Ireland were not erupted at one period, but have been grouped by Prof. 

 Hull into three classes, the oldest of which includes highly silicated felspathic trachytes, 

 porphyry, pearlstone, and pitchstone. The second comprises beds of basic amygdaloid, 

 with bands of bole, volcanic ash, &c. The third is formed of massive sheets of columnar 

 Basalt. A long interval, marked by the changed characters of the lavas, is supposed to 

 have intervened between these stages. Be this as it may, considerable intervals of time 

 are indicated by the boles, lithomarges, and lignites, which must each mark at least an 

 interval sufficiently long for a considerable depth of the compact Basalt to have decomposed 

 into vegetable mould, and remained, perhaps for ages, a soil on which a luxuriant 

 vegetation flourished ; while the lignites and even true coals, six or seven feet thick, 

 found in them, must mark still greater intervals. The eruptions of Basalt were 

 succeeded, at least in Scotland and Iceland, by trachytic flows, which seem to have 

 ended the great period of volcanic energy to which the plants under notice belong.^ 



The enormous period required to build up masses many thousand feet thick, and 

 stretching over 1000 miles in a direct line, may not unreasonably be supposed to represent 

 an entire geological epoch ; but a yet vaster time must have been required to sculpture the 

 once continuous sheets into their present contours. This has not been effected to any 

 appreciable extent by upheavals or dislocations ; but the mountains have been carved out 

 of the solid, inch by inch, by the slow process of weathering, which has cut down 

 through them to a depth of over .2000 feet. River-valleys had been weathered out of 

 them, according to Geikie, before the trachytes of the Scur of Eigg were erupted. The 

 plateau-like aspect, still maintained in the Basaltic area of Ireland, has been lost in 

 Iceland, where denudation has cut them into long rolling hills, separated by wide and 

 nearly parallel valleys. The denudation is colossal, and solitary pinnacles or pro- 

 tuberances everywhere attest, like monuments, the former presence of Basalts, of which 



. 1 Geikie, 'Text-Book of Geology,' 1882, p. 256. 

 ' Tbe present localised manifestations in Iceland are as utterly disconnected from the Tertiary Basaltic 

 outpours as an outbreak in Ireland to-morrow would be. 



