part 2] the skap minor intrusions. 119 



The paucity of dykes exposed, compared with the number that 

 must have been intruded, renders the study of their relationships 

 one of difficulty. When the examples available are determined, 

 not by choice but by chance, there is always the possibility that 

 conclusions formed, though in agreement with existing evidence, 

 may yet be based upon the exception rather than the rule. But 

 the mere recognition of this possibility is in itself sufficient to 

 ensure its due consideration in any attempt to arrive at the correct 

 interpretation of the evidence available. 



South of the granite, intrusions occur in greater force, partly due 

 to the gradual westward encroachment of the Carboniferous Lime- 

 stone outcrop on the north, and partly to the forces operating at the 

 time of injection. The country-rock north of the granite consists 

 of lavas, ashes, and breccias of the Borrowdale Volcanic Series. 

 On the south are the grits, flags, and slates of Silurian age. The 

 general strike of the latter is east-north-east, the direction of the 

 axes of folding. Perpendicular to the folding, and parallel to 

 the faulting accompanying it which has split up the country into 

 somewhat rectangular areas, a narrow belt extending north- 

 north-westwards and south-south-eastwards from the granite forms 

 the principal locus of the dykes. The earlier intrusions trend 

 generally in the same direction. 



As a rule the exposures are not good, and although occasionally 

 a dyke may be traced for a considerable distance across country, it 

 is generally as a series of small outcrops. When more resistant 

 to weathering, as in the case of the acid dykes, prominent ridges 

 may be formed, flanked by deep depressions. The more basic rocks, 

 on the other hand, are prone to weather more easily than the 

 ground-rock and furrows are formed. Spheroidal weathering is not 

 uncommon. The width may be anything from a foot to several 

 yards ; but, while the basic dykes are generally narrow and compact, 

 the acid intrusions are more massive and tend to develop a highly - 

 porphyritic structure. Very rarely can a complete section be 

 found ; the junctions are either invisible or obscure, and the portion 

 available for examination is often extremely limited. 



Singularly little effect has been produced upon the rocks through 

 which the intrusions have been forced. Contacts are very sharp, 

 and, beyond a slight induration extending for a few inches, rarely 

 more than a foot, from the margin, and the occasional formation of 

 fresh biotite; the rocks are scarcely affected. An apparent exception 

 to this will be mentioned later. 



The intrusions usually take the form of dykes, either vertical or 

 highly inclined. There is no general hade, the region having been 

 under the influence of more than one set of movements inducing 

 irregular planes of weakness. The acid rocks form occasionally 

 small irregular bosses, as at Rosgill. Sills are not very common, and 

 occur principally among the intermediate rocks. A sill of quartz- 

 porphyry intruded in the andesitic ashes north of the granite bears 

 some resemblance to the Lake-District intrusions of Ordovician 

 age, but it is probably connected with the granite. 



