Correspondence — Mr. Benj. Smith Lyman. 335 



pointing ont to Mr. Ward, when first he used the term felsidolerite 

 (a term of barbarous etymology and self-contradictory), that the 

 more acid lavas of the Lake District agreed with some porphyrites, 

 and only differed from andesites by slight mineral changes. 



Again, the rhombic pyroxene which I have described in the 

 E3'Oott Hill rock (which I leave among the basalts) differs in some 

 respects from that described by Messrs. Cross, Iddings, and Teall, 

 and (as may be seen from my paper) is more nearly related to the 

 mineral which occurs in certain peridotites and serpentines. As 

 I went in 1876 to examine a rock containing it, I presume it was 

 pretty well known some years previously. It was, however, very 

 natural that Mr. Ward should overlook this mineral — indeed, a 

 characteristic specimen may not have occurred in the slide or slides 

 which he examined. This, however, seems so obvious a truism 

 that the only motive which I can understand in Mr. Eutley's letter 

 is to hint obliquely that I have not done Mr. Ward full justice. 

 This I maintain is not warranted by anything in my paper. No 

 one can regard the memory of Mr. Clifton Ward more highly than 

 I do, for I continue to regret him as a near and dear friend, no less 

 than I esteemed him as a geologist. But I did not and do not 

 consider that I was bound to preface my paper by some apologetic 

 remarks for venturing to correct slightly and add a little to what 

 he had written on the subject. If we, whose lives are spared, are 

 not to endeavour in our humble way to advance knowledge, for 

 what are we living ? T. G. Bonney. 



stjbtereaneajst coxtoueing on geological maps. 



SiK, — In the May Number of the Geological Magazine, received 

 yesterday, a Correspondent of yours asks me : " How the position 

 for contours [of Rock Beds] may be accurately ascertained at depths 

 far removed from observation, amongst highly contorted or disturbed 

 strata ? " Eeally I know of no method but digging ; yet it some- 

 times happens even in such extreme cases that an opinion of more 

 or less value can be formed by means of a careful instrumental 

 survey. 



What I maintain is that contour lines enable any such opinion to 

 be expressed clearly and precisely, of course the mode of expression 

 does not by its clearness and precision increase the certainty or 

 truthfulness of what is expressed. Nor are opinions necessarily 

 valueless because not certain beyond a doubt ; nor are all opinions 

 to be called mere "fancies." If geological maps could literally only 

 give what has actually been observed on the surface, they would 

 generally be barren indeed and leave almost as much to the intel- 

 ligence of the reader as the unexplored ground does. The aim of 

 geological surveys is to ascertain the probabilities in regard to what 

 is hidden ; and in easy cases and thorough surveys the probabilities 

 perceived by the geologist are sometimes px'actically as good as 

 certainties ; in many other cases the indication with precision of the 

 probable, not " merely possible, position " of a rock bed under cover 



