Correspondence. 179 



It seems that '' the petrologist may throw up his hands in despair 

 when he finds Mr. James Geikie describing minette as a quartzless 

 granite." As the pages of this Magazine are probably sometimes 

 scanned by readers who may not be quite familiar with the term 

 minette, but to whom the composition of granite must be well 

 known, I could think of no shorter or more apt description of 

 minette than that given, and cannot see how it is likely to mislead 

 anyone. No one would dream of labelling a museum specimen of 

 minette as quartzless granite, nor of ranging it under the granites 

 in a system of classification. 



My critic further finds fault with my use of the term dioritic, and 

 then proceeds to teach how the petrologist defines diorite and green- 

 stone. In my remark that " under the term dioritic are included all 

 those rocks which consist essentially of silicates of lime and magnesia 

 set in a felspathic base or matrix," I referred only to the rocks 

 alluded to in my paper as characteristic of the district described, 

 viz., the diorites, hyperites, etc. ; nor can I imagine how I should 

 have been understood to mean more. The closely allied nature of 

 hornblende, hypersthene, and diallage seemed to warrant me in using 

 dioritic as a convenient general term for the rocks in which those 

 minerals make their appearance.^ 



From the tone of Mr. Forbes' remarks one might gather that the 

 terminology of petrology was as fixed as that of the exact sciences. 

 Scarcely two petrologists, however, can be found to agree in their 

 definitions of many rocks. " The petrologist," we are told, " regards 

 greenstone as that variety of diorite in which green or dark coloured 

 hornblende either predominates, or, when the rock is fine-grained, 

 renders more obscure the presence of the felspar." Now the term 

 greenstone has long been employed by writers on Scottish geology as 

 a generic and not a specific term. Hence we read of hornhlendic 

 as distinguished from augitic greenstones.^ As it is sometimes im- 

 possible to tell in the field whether a rock is to be classed as a 

 diorite or dolerite, the use of greenstone as a generic term has proved 

 of some utility.^ 



It appears to be "difficult for a petrologist tO' understand what a 



• Cotta includes hyperite, diorite, diallage-rock, and some other allied rocks in his 

 Greenstone group. 



* The term augitic greenstone is not, however, confined to the pages of writers on 

 Scottish geology; Cotta has the same expression. [See " Eocks classified and 

 described," p. 146.] He describes the "greenstones" as "compounds of some species 

 of felspar with pyroxene, or hornblende, as essential ingredients, etc. : " among the species 

 of greenstone augite-porphyry is mentioned. In Professor Phillips's " Manual " we 

 find mention made of atigitic greenstone (augite and felspar) as distinguished from 

 greenstone (hornblende and felspar) ; and Sir C. Lyell, while he defines greenstone to 

 be a compound of felspar and hornblende, yet takes care to state that " the name has 

 usually been extended to all granular mixtures, whether of hornblende and felspar, or 

 of augite and felspar." (Elements of Geol., p. 594). 



3 J'emploie le terme de griiiistein pour les roches trappeenes cristallisees verdatres, 

 que je nai pas examinees, et qui ont ete decrites sous ce nom, et pour celles que j'ai 

 bien observees en place, mais dont je n'ai conserve aucun echantillon pour pouvoir 

 constater k present si ce sont dcs dolerites ou des diabases." Boue, Essai Geologique 



HUT VEcosse, 1820, p. 135. 



