Correfipondence — Dr. W. T. Blanford. 239 



are merely possible positions, amongst many others at whicTi a con- 

 cealed stratum might be found. 



This element of uncertainty, it is true, affects the absolute accuracy 

 of many geological sections, if not of most — in an unavoidable vvay, 

 for which reason it would be wiser to reduce than to multiply sources 

 of error. 



A geological map is in a sense pictorial, and the more so the more 

 easily understood : competing interests destroy such pictures as 

 contain them by creating confusion ; hence, is it not better to struggle 

 on with existing difficulties in order to convey surface observations 

 intelligibly, than seek to overlay our maps with a complex of sub- 

 terraneous assertions — save where necessity and data may both exist, 

 as in the case of mining plans for special purposes. 



Supplementing facts with fancies may possess a charm, but the 

 tendency is dangerously apt to degenerate, or lead us into paths 

 which geological observers of orthodox principles are as yet not 

 imperatively called upon to tread. 



Lest these remarks should be considered captious, let me express 

 a hope that Professor Lyman will further favour your readers with 

 information as to how the positions for contours may be accurately 

 ascertained at depths far removed from observation, amongst highly 

 contorted or disturbed strata? And also as to whether he would 

 advise the use of distinct plans on which to record the positions of 

 the contours at the various depths, when ascertained. 



Kingstown, March llth, 1885. 



THE CLASSIFICATION OP THE JURASSIC SYSTEM. 



Sir, — If Mr. Jukes-Browne is satisfied with the argument that 

 because a lithological change does take place, in England, France 

 and Germany, about the line of division between the Cornbrash and 

 Oxfordian, therefore this is a good line of separation between Middle 

 and Upper Jurassics, and one with which most English geologists 

 will be satisfied, I fear it is useless for me to argue further. The 

 statement about the lithological change is true in a certain sense, 

 but it is one of those unscientific half-truths that ignore the main 

 facts. Amongst the facts ignored in this case are the following : 

 The lithological change in Germany from the argillaceous beds of 

 the Brown jura below to the calcareous strata of the White Jura 

 above is precisely the reverse of that wliich takes place in North- 

 Western France and England, and the horizon where the change 

 takes place is not the same, in fact the two changes have no con- 

 nection with each other. So purely local are the lithological con- 

 ditions on which Mr. Jukes-Browne relies that the argillaceous Upper 

 Jurassics of England and North-Western France are represented even 

 in Central France by calcareous beds. 



I did not attempt to enter into the classification of minor sub- 

 divisions like the Lower Calcareous Grit. But when Mr. Jukes- 



