350 HARKER : IGNEOUS DYKES OF THE NORTH OF ENGLAND. 



prevailed throughout the time when the rocks of the Borrowdale Series 

 were laid down, and we may therefore expect to find dykes of this age 

 in the same district. The most imposing manifestations of vulcanicity 

 belonging to this age are the andesitic lavas and agglomerates so 

 largely developed in the central zone of the Lake District, forming 

 the greater portion of the Borrowdale Series of rocks. Dykes of 

 similar (i.e. 'intermediate') composition do indeed occur intersecting 

 the Skiddaw Slates at certain places; but since the sites of the 

 andesitic volcanoes are not certainly known (Mr. Clifton Ward 

 considered the Castle Head, Keswick, to mark one of these), it is 

 impossible to connect the dykes definitely with this period of 

 volcanic activity. It is of course to be borne in mind that, owing 

 to their conditions of consolidation, the rocks of the larger dykes 

 would take the structure of dolerite or diabase rather than andesite. 



The andesitic rocks were succeeded by an out-pouring of acid 

 lavas (rhyolites), which are now well seen along the southern border 

 of the former series. The volcanoes which emitted these newer 

 lavas are long since destroyed ; but there are good reasons for 

 supposing that their basal wrecks are still represented by some of 

 the masses of crystalline acid rocks, which break through, especially 

 on the boundary of the Skiddaw Slates and the Borrowdale Series, 

 in St. John's Vale, in Ennerdale and by Buttermere, and perhaps on 

 Caldbeck Fells in the north. If so, this fixes the date of numerous 

 dykes of micro-granite, granophyre, and quartz-porphyry which may 

 credibly be linked with the masses in question. For instance, the 

 dykes of spherulitic quartz-porphyry near Armboth and Helvellyn 

 seem to belong to the microgranitic boss of St. John's Vale : others 

 in Ennerdale are doubtless connected with the granophyre of the 

 Ennerdale mass. A number of dykes of granite and quartz-porphyry 

 to the south of Shap Wells may be related to the porphyritic granite 

 boss close by ; but these must be of later date than the others 

 mentioned, for the Shap Wells boss breaks through the Coniston 

 Limestone Series, which overlies the Borrowdales, and perhaps 

 through higher strata. This invasion of acid rocks must, however, 

 have taken place during some part of the Silurian period proper, or 

 immediately after it, for pebbles of the well-known Shap Granite are 

 found in the basal conglomerate of the Carboniferous, the Old Red 

 Sandstone of the earlier writers. Again, granite and quartz-porphyry 

 dykes on Kirkfell and near Bootle seem to be allied to the great 

 granitic mass of Eskdale and Wastdale ; but there is nothing to 

 connect this system of intrusions definitely with the rhyolitic lavas. 

 Perhaps, as first suggested by Sedgwick, the injection of the molten 

 matter was connected with the folding and disturbance of the area 



Naturalist, 



