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THE IGNEOUS DYKES OF THE NORTH 

 OF ENGLAND. 



ALFRED HARKER, M.A., F.G.S., 



Fellow of St. John 's College, Cambridge. 



Among the minor phenomena of field geology, few features are more 

 striking than the various igneous dykes of which the British Isles 

 afford such numerous examples. Ranging as more or less vertical 

 walls through stratified or other rocks of totally dissimilar character, 

 and often persisting for considerable distances in a straight line with 

 approximately uniform width and constant lithological characters, 

 they are sufficiently distinctive to be at once recognised when found. 

 Often they stand out slightly above the adjacent rocks at the surface, 

 owing to superior durability ; in other cases they have weathered 

 more rapidly than the 'country rock,' and figure in the landscape as 

 slight depressions or trenches ; in some places, again, small dykes are 

 liable to be passed over unless carefully sought. 



In many modern volcanoes the filling of fissures with molten 

 lava, which solidifies into dykes, is seen to be a common accom- 

 paniment of volcanic action. Many of the dykes exposed among 

 the rocks by denudation appear, on the other hand, to be offshoots 

 of deep-seated masses of igneous material. They have been 

 injected under great pressure from below, and in most cases probably 

 never reached the surface of the earth. Again, it appears that dykes 

 have served as the channels by which molten matter on a very large 

 scale has been forced upward and extruded over the surface in 

 massive lava-flows. By Richthofen and others some of the grandest 

 out-pourings of lava in Tertiary and possibly in earlier times have 

 been ascribed to these 'fissure-eruptions' rather than to volcanic 

 vents of the ordinary type. 



In the nature of the case, it must be extremely difficult to obtain 

 direct ocular evidence of the connexion of dykes in partially 

 denuded districts, either with plutonic bosses or reservoirs, or with 

 volcanic activity long dead. It seems probable, however, that much 

 may yet be done in this direction by detailed work — by noting the 

 localities of dykes (many of which are known, but doubtless many 

 more unknown), and by observing their bearing, their width, and 

 especially their petrological characters. 



The earliest dykes in the North of England must naturally be 

 looked for in the Lake District, where the oldest strata in this part of 

 the country crop out. Here volcanic conditions are known to have 



Dec 1888. Y 



