Correspondence — Mr. Daniel Jones. 575 



the charge of which I was intrusted with by Mr. Hartley, one of the 

 Commissioners. 



I regret that my evidence on the co-relation of the Shropshire and 

 Staffordshire Coal-fields is not published in detail in vol. ii. Prof. 

 Eamsay has given an importance to it by his remarks (p. 121, vol. i. 

 Coal Commission Report) which it might not otherwise have enjoyed. 

 I hope shortly to arrange for its appearance in print, so that the 

 reader may judge for himself of the reasonableness of my arguments. 



Albrighton, near "Wolverhampton, t~» t 



Mvember 8, 1871. Daniel JoNBS. 



EELATIVE AGES OF IGNEOUS EOCKS. 



Sib, — On my return from Italy a few days since, I had the pleasure 

 of reading Mr. Allport's article, '■' On the Eelative Ages of Igneous 

 Eocks," in the Geol. Mag. for October, p. 448. I am obliged to him 

 for directing my attention to an error into which I was led by quoting 

 from my impression of the results of Mr. Forbes's researches on the 

 microscopical structure of basalts, and if I have incorrectly repre- 

 sented them, I sincerely apologize. 



I am glad, however, that I have correctly stated the result of Mr. 

 Allport's own observations, with which I was well acquainted 

 through the valuable papers he has contributed to this Magazine, 

 and I take this opportunity of thanking him for the ready assistance 

 he afforded myself when commencing the microscopical examination 

 of rocks. As he has shown, olivine, which was once considered as 

 characteristic of Tertiary or modern basalts, is present either in its 

 primary form or by its pseudomorphs in melaphyres, dolerites, and 

 basalts of various geological epochs ; and with reference to Dr. 

 Zirkel, whose name I ventured also to quote, I have it on very good 

 authority that he has arrived at a similar conclusion. 



My own observations on the microscopical structure of basalts and 

 dolerites are not sufficiently extended to enable me to come to any 

 certain conclusion as regards the presence of olivine, or the struc- 

 tural peculiarities of rocks of different ages. It is, in the first place, 

 often quite impossible to determine to what geological period or 

 epoch a trap-dyke may be referable. In the case of the Mourne 

 Mountains we have an unusual means for the determination of the 

 relative ages of two sets of basaltic or doleritic dykes, owing to 

 their relations to the granite ; and we cannot be far wrong in as- 

 suming that the dykes more recent than the granite are referable to 

 the Tertiary epoch, and that those more ancient than the granite are 

 of Upper Carboniferous, or possibly Permian, age. Now, on referring 

 to my notes of the few specimens I have been able to examine 

 microscopically from this district, I find that the crystalline grains 

 of olivine, or its pseudomorphs, are (as far as I can determine) only 

 present in the more recent dykes. The number of specimens is, 

 however, quite insufficient for any general conclusions to be founded 

 on them. 



In conclusion, allow me to express a hope that Mr. Allport will 

 not be content with publishing a few papers on the interesting 



