/O Marker : Petrology in Yorkshire. 



and for the aid which it can be made to afford to many other 

 problems of general geological interest. 



To discuss the specific results already obtained, relative 

 to the dispersal of travelled boulders at different stages of the 

 glaciation, would carry me beyond the limits which I have laid 

 down for myself in this address. Enough that the results 

 demonstrate conclusively the value of concerted and systematic 

 research addressed to a definite end. Local in its inception and 

 in its immediate interest, the investigation is one which neces- 

 sarily oversteps the boundaries of the county and indeed of the 

 British Isles. Briefly, the problem is to trace the various 

 types of rocks met with among the boulders to their several 

 homes in the North of England, the South of Scotland, and the 

 Scandinavian peninsula. This necessitates, not only a study 

 of the boulders themselves, but a sufiicient acquaintance with 

 the rocks of all those districts which can possibly have con- 

 tributed boulders to our drift deposits. Such areas must 

 be visited for the purpose of procuring representative collec- 

 tions to serve as standards of comparison. Our Yorkshire 

 workers have not been daunted by this comprehensive pro- 

 gramme, and to the thoroughness thus evinced is due the 

 special value which belongs to the results obtained in recent 

 years. It is therefore in no critical spirit, but in one of hearty 

 appreciation, that I speak of this interesting application of 

 petrology in the service of glacial geology. 



Among the great diversity which the travelled boulders 

 of East Yorkshire present, special importance attaches to 

 strongly marked types which are known to be unique, or at 

 least of very restricted distribution in situ. There are many 

 of these, mostly igneous rocks, but they usually make up only 

 a fraction of the whole assemblage. The rest, belonging to 

 less distinctive and sometimes widely distributed types, are 

 of less significance, though they have their weight as cumulative 

 evidence. There are, for instance, large numbers of gneissic 

 and other crystalline rocks which may safely be referred to 

 some Scandinavian source, merely because there is no other 

 area of similar rocks which enters into the probabilities ; but 

 it is clearly much more satisfactory when we can assert con- 

 fidently that a certain boulder has once formed part of a 

 particular hill in Norway. Among the well-defined types which 

 furnish the most precise information, some are easily recognised 

 at sight. Nobody can mistake a boulder of Shap Granite or 

 of rhomb-porphyry ; but there are many other rocks, equall}^ 

 useful in tracing the movement of the ice, which cannot be 

 safely identified without microscopical examination. In every 

 such case, I think, the record should state whether this means 

 has been employed, and also who is responsible for the identi- 

 fication given. If only five records in a hundred are doubtful, 



