>S'. AUport — Relative Ages of Igneous Rocks. 449 



criterion of age amongst the constituents of basalt, dolerite, or mela- 

 phyre." 



In the first place, I think Mr. Forbes will object to be placed in 

 this connexion, as he has not long since expressed a very decided 

 opinion that " eruptive rocks of identical mineral constitution have 

 made their appearance or intrusion into the earth's crust at similar 

 geological epochs ; " and, conversely, that " when the geological 

 epochs of the appearance of two or more intrusive or eruptive rocks 

 are known to differ, these rocks will then also be found to differ 

 essentially in mineral constitution." ' Then with respect to Prof 

 Zirkel, I am not aware that he has anywhere stated that the old 

 melaphyres and more recent basalts are mineralogically the same ; 

 in fact, in his work on the basalts, published in 1870, he saj'^s, in an 

 appendix on melaphyre,^ " the structure and mineralogical constitution 

 of these apparently compact rocks still remains in the greatest 

 obscurity ; " and then gives the results of the examination of a few 

 specimens, reserving definite conclusions on their composition, etc., 

 till further evidence is forthcoming. It is clear, however, that he 

 perceived a close resemblance between the two series. 



Now it would appear from the above quotations, that my investiga- 

 tions have led me to conclusions completely at variance with those 

 arrived at by Mr. Forbes, and I am not aware that others have 

 shown that there is no essential difference between eruptive rocks of 

 difi'erent geological epochs. I arrived at this conclusion more than 

 three years since, as the result of a microscopic examination of 

 very many sections prepared by myself, and I am under the im- 

 pression that I have been the first to ascertain the fact. 



I have abundant evidence that melaphyres of undoubted Carbon- 

 iferous age, and basalts of Tertiary age, have not only the same 

 mineral constitution, but also that both present the same structural 

 varieties. 



A highly characteristic structure in many basalts is that in which 

 the various constituents form a close net- work of crystals in actual 

 contact with each other, without any intervening cement ; in others, 

 there is a vitreous, or semi-vitreous base, in which the crystals are 

 imbedded ; or, in many, the rock is composed of a mass of very 

 minute crystals, in which larger ones are porphyritically imbedded. 



Now it is an interesting fact, that these varieties of texture are 

 equally common in the older melaphyres, affording, I think, addi- 

 tional evidence that both were formed, not only of similar materials, 

 but also under like conditions. There is, in fact, no lithological or 

 petrological difference whatever between them, for both agree com- 

 pletely in composition, texture, and modes of occurrence. 



The statement that there is no mineralogical difference, will, of 

 course, be understood to apply to the least altered portions of the 

 older rocks, for the latter have generally undergone a very consider- 

 able amount of alteration ; but the difference thus set up between 

 them is one which may be readily recogiiized as due to chemical or 



1 Researches in British Mineralogy, Phil. Mag., vol. xxxiv. p. 336. 

 ^ TJntersuchungen iiber die Basaltgesteine, p. 198. 



VOL. VIII. — NO. Lxxxvm. 29 



