120 A. W. Jackson— Xon unci atvr< of Crystalline Rocks. 



certainly expect dprfori that they would have been formed then 

 as now ; but in the tremendous lapse of time that has intervened 

 they would necessarily have become either greatly mo 

 form by erosion or in appearance by subsequent metamorphism 

 so as to become unrecognizable as such, or indeed entirely ob- 

 literated. It would require peculiarly favorable conditions to 

 preserve a Paleozoic tufa it) a recognizable condition to the 

 present day. On the other hand, it is now conceived that 

 irranitic rocks arc forming in the subterranean depths at the 

 [".resent time. The differences that do exist between so-called 

 '■older"' and ''newer" rocks may be due then not to their 

 _ but to ! « rem i j -ical condition? 



of their crystallization, conditions which held equally in Paleo- 

 zoic times and in Tertiary times, only the Paleozoic superficial 

 have been mostly swept away and the deep seated 

 rocks of that age exposed by the removal of thousands of feet 

 of superincumbent strata while the superficial rocks of Post- 

 Ci ' eous times, the tra hyi -. andesites, phonolites, basalts, 

 etc., are preserved to us because of their recent origin and the 

 granitic rocks now /arming are too deep-seated to be exposed 

 for observation. 



I would oppose then this distinction of rocks into "older"' 

 and " newer " (for purposes of nomenclature) first, because the 

 original grounds upon which it was made seem no longer ten- 

 able: second, because such distinctions as exist in fact between 

 older and younger rocks can be accurately expressed by a 

 purely mineralogical nomenclature; third, because the distinc- 

 tion is not and cannot be consistently carried out between rocks 

 of Mesozoie and Paleozoic age ; fourth, because of its great in- 

 convenience. It is not always possible to tell the age of a rock; 

 ame must be held in abeyance or at least 



subject to change. More than one instance will suggest ; 



to the specialist where the name of a rock supposed "to be Pre- 

 Tertiary has been changed when subsequent study in the held 

 has proved the rock to be Tertiary or Post-Tertiary. This 

 element of uncertainty plays sad havoc with rock-names and 

 should be set aside entirely. 



The third geological consideration to be noticed is origin. 



The first objection to be urged against its use in rock-naming 



I of rock-genesis lies too much 



exact mode of formation, even if we could attach definite con- 

 ceptions to such terms as "metamorphism," "aqueo-igneous 

 fusion," etc., the objection similar to that urged against "age" 

 would still remain, namely, rocks with the same minei 



