STRUCTURE OF ERUPTIVE ROCKS 211 
still recognisable in the great necks and bosses which have 
been exposed by denudation. In other cases of widely 
extended effusive rocks, we appear to have the products of 
vast fissure-eruptions. Of such a character are the plateau- 
basalts of the Western Islands of Scotland, Antrim, the 
Fzroe Islands, Iceland, etc. At the time of these eruptions, 
the whole wide region extending from the British Islands to 
Greenland appears to have been underlaid by a vast sea of 
molten matter, which rose to the surface along rents in the 
crust and deluged the surrounding areas with floods of lava. 
Such rents and fissures were doubtless the result of earth- 
quake action; and many of them did not reach the surface, 
dying-out upwards at various levels. Into these, however, 
molten matter found its way, forming the great series of 
basalt-dykes shown in the map, Plate X_LVIII.* 
Sandstone Dykes.—Here brief reference may be made to certain 
abnormal dykes, occurring in California and elsewhere in North America. 
They are composed of sedimentary materials, and occupy vertical fissures, 
which have been filled not from above but from below. Some of these 
‘dykes have a length of several miles, and their precise mode of origin is 
obscure. The sand may have been introduced from below during earth- 
quake movements. For unconsolidated materials, such as water-logged 
clay and sand, when buried under a considerable thickness of super- 
incumbent rock, are ready to rise towards the surface along any open 
fissures that may be formed. Occasionally boring operations in our coal- 
fields have been impeded in this way by the more or less rapid rising in a 
bore-hole of soft clay, coming from a considerable depth. 

* It ought to be mentioned, however, that some of the dykes shown 
upon the map date back to much earlier periods. For example, certain 
dykes traversing the Carboniferous tracts appear to be of late Carboni- 
ferous age. 


