22 SCIENCE PROGRESS. 



Anglesey and in W^icklow Sir A. Geikie has identified 

 numerous ancient vents, apparently of late Ordovician age, 

 filled for the most part with masses of agglomerate. In 

 Carnarvonshire erosion has exposed the relations at a 

 deeper level, and the sites of the old volcanoes are now 

 marked by intrusive bosses and plugs. Here and in similar 

 cases the scarcity of true dykes seems to be a characteristic 

 feature. 



The last-named district is sufficient in itself to illustrate 

 many features which seem to be characteristic of an evolu- 

 tion of an area of igneous activity, and, in particular, it 

 exhibits with exceptional clearness the manner in which 

 the several stages are bound up with the local crust- 

 movements which brought the Ordovician period to a 

 close. The linear disposition of the old orifices of eruption 

 is well displayed on a geological map of the district. The 

 line runs south-westward from near Penmaenmawr far down 

 into the Lleyn peninsula, parallel to the axes of folding, and 

 occupying precisely the position it should have if the erup- 

 tions were a consequence of the increasing thrust from the 

 south-east (3). It will, of course, be observed thg-t from our 

 present point of view the frequent linear arrangement of vol- 

 canic vents, to which we adverted at the outset, connects itself 

 directly with the distribution of stresses in the earth's crust, 

 visibly expressed in axes of folding, etc. Whether or not 

 the vents are situated upon actual lines of fissure is not an 

 essential question, and the coincidence of many lines of 

 volcanoes with coast-lines is equally a secondary feature. 



The part played by so-called " massive " or "fissure" 

 eruptions in the volcanic history of a region is scarcely to 

 be estimated without more precise knowledge of their 

 nature and characteristics than we at present possess. 

 Some great out-pourings of lava which have been regarded 

 in this light, such as the basalts of the Snake River plains, 

 are certainly very late occurrences. If the numerous dykes 

 which cut the Archaean of the north-west Highlands of 

 Scotland have been the channels of fissure-eruptions, these 

 would seem to have been amonsf the latest manifestations 

 of pre-Torridon igneous activity ; though, if we connect 



