part 2] THE SHAP MINOH INTEUSIOXS. 187 



A further point may be noted in this connexion. The more 

 acid of the mixed series on the north and the more basic on the 

 south, can be correlated with a similar greater acidity north of the 

 Skiddaw granite. The plutonic masses themselves are similarly 

 related. All appear to be connected with the crustal stresses which 

 produced the present strike of the Lake- District rocks. 



The true explanation of the comparative basicity of the Shap 

 granite may lie in a combination of these 1 wo factors. Pressure 

 from the south-east affecting first and in full force the Shap area 

 might conceivably produce a considerable flow of material to the 

 existing surface, while the more northern magma, less deeply 

 stirred by a partly-spent force, failed to reach the surface, and 

 consolidated as a laccolite. In each case the continued pressure 

 forced the more acid portions of the magma northwards. 



YI. Age and Order ok the Intrusions. 



Two points require preliminary consideration : (1) the age of the 

 granite; (2) whether all the minor intrusions in the area belong 

 to the- same igneous epoch. As regards the former, the age can 

 with certainty be taken as Devonian, the evidence on this point 

 being conclusive. The second is perhaps more difficult to deter- 

 mine, but a genetic connexion is highly probable. All available 

 evidence supports it. Dr. Harker has given reasons for believing 

 that such a genetic connexion exists between the lamprophyres 

 and the granite magma. 1 Similar views have been expressed 

 regarding the dykes and sills of the adjoining area by Sir Aubrey 

 Strahan, who regards them as belonging to the same outburst as 

 the Shap granite.- 



All the dykes examined agree in certain respects. They are 

 intruded in Silurian or earlier rocks, and are not known to enter 

 Carboniferous rocks. In the central part of the area the general 

 direction of the dykes is north-north-westerly, corresponding 

 approximately to the fractures transverse to the folding, important 

 deviations apart from sills being due, as a rule, to intrusion along 

 lines of fault. A belt in this direction encloses practically all the 

 orthoporphyritic intrusions, together with certain other acid rocks. 

 Outside this belt on the west are the intermediate rocks of the 

 Potter-Fell type, and at a greater angle the most basic of the 

 lamprophyres. 



Xone of these dykes existed, so far as is known, in Ordovician 

 times. They have not suffered from the earth-movements pro- 

 ducing the folding, and do not show signs of crush or shearing. 

 Even when rocks intruded in Silurian sediments lack alignment, 

 as in Faweett Forest, where the sills exposed on the ridges are 

 apparently shifted in succession to the north-west along lines 

 of fault coincident with the intervening valleys, crush-phenomena 



1 Geol. Mag. dec. 3, vol. ix (1892) p. 199. 



2 'The Geology of the Country around Mallerstang, &c.' Mem. Geol. Surv. 

 1891. p. 8. 



