212 SCIENCE PROGRESS. 



The nature of the intrusion itself and its relation to folding 

 were made clear by Lossen (19) in his researches in the 

 Harz district, and have been generally recognised. Ex- 

 amples of lenticular intrusive bodies of diabase following 

 the bedding of the strata and clearly determined by folds 

 have been pointed out by Watts at Corndon and other 

 places in the Shropshire district. In some parts of North 

 Wales the maps of the Geological Survey show very evi- 

 dently how the diabases have been injected mainly along 

 the crests and troughs of the folds, that is, in the places 

 where the strata would tend to part in consequence of the 

 lateral thrust. Here the individual intrusions attain no 

 great thickness, being rather of the nature of inconstant 

 sheets. Regarding the more massive lenticles as the ana- 

 logue in a mountain-region of the typical laccolite, these 

 more attenuated forms correspond with those which in a 

 less disturbed region appear as regular sills. Among folded 

 strata, however, the relative thickness and extent of an in- 

 truded mass cannot have the significance which has been 

 ascribed to them in a plateau region, being determined by 

 externally impressed forces rather than by the fluidity of 

 the magma or the depth of cover. It should be observed 

 too that the surfaces of weakness in the strata which guide 

 the injected magma are not always those of bedding, but 

 may be determined by faults, overthrust or underthrust 

 faults, or other structural accidents. 



The varied bodies of intrusive igneous rock hitherto 

 referred to are of well defined and clearly ascertainable 

 forms, but the same cannot be said of the masses, often of 

 considerable size, known as dosses or stocks and consisting 

 generally of rocks of plutonic characters. These large in- 

 trusive masses are, as a rule, clearly transgressive as regards 

 their exposed upper surfaces, but the form and relations of 

 their deeper parts are necessarily matters beyond direct 

 observation, and very divergent opinions have been held 

 concerning them. Some geologists have considered a boss, 

 say of granite, as exposed at the surface, to be merely the 

 summit of a great mass, pictured as of roughly pyramidal 

 shape, extending downward with increasing horizontal di- 



