194 



PEEIDOTITE. 



and altered forms by the respective varietal terms, tufa and porodite, or, if 

 desirable, their adjective forms, iiifaceous and poroditlc. 



Below is given a table showing the classification of all the rocks more 

 basic than the basalts, so far as now known, but it admits of the ready addi- 

 tion of other varietal names, and even of specific ones, if future studies shall 

 indicate such divisions. The three designated classes of varieties are of course 

 not entirely distinct, since in alteration a mineralogical change enters, and 

 that change or alteration is the basis of the two divisions of the fragmental 

 forms, while these may partake of the mineral characters of the mineralogi- 

 cal varieties, and belong to them. 



Tabic shoiving the Classification of the Hock Species preceding the Basalts. 



This seeming confusion or ci'ossing of names is due to the present state of 

 lithology, and the necessity of bringing the nomenclature into accordance with 

 the general usage and prejudices of lithologists, since an abrupt departure 

 from their nomenclature would only repel, and the inconsistencies can later be 

 remedied by the dropping of all varietal names which the advance of the 

 science shall render superfluous, as the writer now believes many of tliein to 

 be. Thus, for instance, it is here considered that in peridotite, the terms 

 pcrkloiitc, serpentine, and talc and actinolite schist, with the adjectives iifaccous 

 and poroditlc, will express every essential form of these I'ocks now known, 

 and by their use alone a proper conception of their relations would be ad- 

 vanced. 



