6q 



PETROLOGY IN YORKSHIRE. 



ALFKICD HARKER, M.A., F.R.S., F.G.S., 

 Camhi-idge. 



{^Continued from- page 44). 



I will not weary you by enlarging upon numerous other 

 subjects in which work still remains to be done, such as the 

 oolitic structure and its possible connection with organic agency, 

 or the flints in the Chalk, especially in relation to disseminated 

 silica in the rock. The minute constitution and microzoology 

 of the Chalk itself afford a subject which is far from being 

 exhausted. It may be remarked that, while the soft chalk of 

 the South can often be studied only by rubbing down in water, 

 much of our Yorkshire chalk is hard enough for the preparation 

 of thin slices. The non-calcareous portion of our Jurassic 

 limestones, obtained as a residue on dissolving the rock in 

 dilute acid, would doubtless reward a systematic study. By 

 ' systematic ' I mean especially comparative, both as regards 

 the different members of a formation and as regards the lateral 

 extent of any one member. A large-scale map, on which to 

 record results, is a useful adjunct to an investigation of this 

 kind. 



There remain the igneous rocks. The red colours conse- 

 crated by usage to these figure, it is true, very sparingly on 

 the geological map of Yorkshire. The well-known Cleveland 

 dyke and the Whin Sill have been exhaustivel}^ studied ; but 

 the same cannot be said of the mica-lamprophyre and allied 

 dykes which intersect the Lower Palaeozoic strata wherever 

 these are exposed, in the Ingleton and Sedbergh districts, and 

 in Upper Teesdale. The volcanic rocks intercalated in the 

 Coniston Limestone group of the Sedbergh district have not 

 yet received any detailed notice. Nevertheless, we must admit 

 that, in this direction, the field appears a very restricted one. 



The prospect is ver}' difterent, however, if we take into 

 account the glacial boulders which are found in such profusion 

 and variety embedded in the boulder-clays, scattered as 

 erratics over the surface, and accumulating on the sea- 

 beaches from the gradual waste of the clay cliffs. Li East 

 Yorkshire, if anywhere in the world, the student of igneous 

 rocks need be at no loss for material ; while he has at the same 

 time the opportunity of rendering valuable assistance to 

 glacial geology. Nor has this opportunity been neglected. 

 Here at least our county need fear no reproach ; rather may we 

 justly claim for it the credit of leading the way in this particular 

 line. The impetus has come perhaps rather from the glacial 

 than from the petrological side ; but this only emphasizes the 

 solidarity of geology throughout its different ramifications ; and 

 we may hope that he who has been drawn to petrology for the 

 sake of one of its applications will adhere to it for its own sake , 



igi2 Mar. i. 



