MARKER: IGNEOUS DYKES OF THE NORTH OF ENGLAND. 35 1 



at the close of the Silurian period, and the same conjecture is 

 plausible for the granite of Shap Wells and its attendant dykes. 



Certain groups of dykes in the Lake District show a parallel 

 disposition in evident relation to the strike of the rocks in which 

 they occur. These must clearly be due to more widely operating 

 agencies than are the dykes which show no special disposition, or 

 radiate in rude order about plutonic bosses. The red felspathic 

 dykes to the west of Windermere and some of those to the north of 

 Kendal, although decidedly cutting across the strata, run parallel to 

 the strike. It follows that these dykes were injected at a time 

 posterior to the disturbances which gave the beds their present 

 direction of dip, that is, later than the close of the Silurian, though 

 not necessarily much later. 



A very remarkable group of dyke-rocks is that of the lampro- 

 phyres or 'mica-traps,' so-called from their large proportion of dark 

 mica, occasionally replaced, however, by hornblende. They may be 

 more minutely classed as minette, kersantite, vosgesite, etc., and 

 a number of them have been described by Prof Bonney and 

 Mr. Houghton. Such rocks are largely represented among the 

 dykes seen east of Windermere, to Kendal, Sedbergh, and Dent; 

 they appear also in Swindale and Knock Becks to the north-east of 

 Appleby, in Ingleton Dale, and in the little Silurian inlier discovered 

 m Teesdale by Messrs. Gunn and Clough. From their occurring in 

 all the Silurian strata, not infrequently occupying dip-faults, but 

 never penetrating into the Carboniferous, the mica-traps may safely 

 be referred to the Old Red Sandstone period. Owing to their 

 readily destructible nature, and often their small width, they are not 

 always easy to detect. 



Going now to the far North, we find in the Cheviot district 

 certain quartz-porphyry dykes breaking through andesitic lavas of 

 Old Red Sandstone age. ^h. Teall, who has given a valuable 

 description of all the igneous rocks of this district, regards these 

 intrusions as probably connected in origin with the Cheviot granites, 

 and as constituting the last phase of one general system of igneous 

 outbursts during Old Red Sandstone times. The order of succession, 

 acid rocks following those of intermediate comj^osition, is one 

 which can be verified in numerous other districts, and which lends 

 itself to some interesting speculations. 



There is no very clear evidence of the injection of igneous dykes 

 in tlie North of England during the Carboniferous period, although 

 the 'toad-stones' (olivine-dolerite lavas) of Derbyshire indicate 

 a certain amount of contemporaneous volcanic activity. It is 

 probably to the close of this period that we must refer the intrusion 



Dec. 1888. 



