1883.] 419 [Wads worth. 



In objecting to the separation of rocks into distinct species 

 according as they are pre-Tertiary or not, I am on common ground 

 with Dana, Reyer, Allport, and many others, especially the Eng- 

 lish geologists ; but since Zirkel refers to his classification pub- 

 lished in 1873 as proof that he has always worked on this principle, 

 a few extracts will be given from his work to which he refers. 1 

 He says in substance : " The common division of the eruptive 

 rocks into older (pre-Tertiary) and younger (Tertiary and post- 

 Tertiary) and the nomenclature dependent thereon appear to be 

 of little value. Leaving out of account the variable microstruc- 

 ture of the quartz and the alteration of the feldspars, there is ab- 

 solutely no distinction between the older quartz porphyries [f el- 

 sites] and the younger liparites [rhyolites]. If we do not consider 

 the more or less common molecular alteration, there exists no 

 difference either in composition or structure between many old 

 melaphyrs and younger basalts. The carboniferous diabase and 

 miocene basalt of Scotland are microscopically undistinguishable 

 from one another. It is to be hoped that those rocks which are 

 identical in mineralogical composition and structural peculiarities 

 will be designated by a single name. The age differences which 

 are conditional upon certain small variations can be adjectively 

 expressed. Why should not the same principle of nomenclature 

 be adopted for the eruptive as for the sedimentary rocks. We 

 have Carboniferous and Tertiary limestone and sandstone, why 

 not Carboniferous and Tertiary basalt or diabase . . . Indeed, 

 the combination of plagioclase and diallage with or without oli- 

 vine is known as gabbro with an entire disregard of the principle 

 of age, whether it belongs to the carboniferous ; or, as in Italy, 

 it is an eocene eruption ; or, as in the Hebrides, it is only a geo- 

 logical dependence on the miocene basalt." I might continue 

 quoting from Zirkel's writings similar remarks, but enough has 

 been given to show that my views are but the logical application 

 of that which he has admitted to be true. 



To maintain, as most classifications must do to have any basis, 

 that the products of fissure and volcanic eruptions entirely changed 

 between the Cretaceous and Tertiary periods, and that no species 

 previous to the Tertiary have ever been produced since, even if 

 the rocks are absolutely identical in every particular, is in my 



l Mikros. Beschaf. Min. Gest., pp. 291- 93. 



