558 F. Ridley — Igneous Rocks of the Warwickshire Coal-field. 



are often wanting, and it is seldom that any unaltered olivine is 

 present. The replacing substance is frequently calcite. Accepting, 

 therefore, the conclusions of Allport, who was fortunate in finding 

 good crystals of unaltered olivine in some of the dioritic rocks of 

 this district, and coupling them with such evidence of the former 

 presence of this mineral as the sections, here described, afford, there 

 seems little doubt that we have, in some cases, rocks which may 

 often, with equal justice, be termed either olivine-bearing diorites or 

 hornblendic basalts. On the other hand, the association of augite 

 with hornblende in some of these rocks, also described by Allport, 

 gives rise to a like ambiguity, since in these we may have either 

 hornblendic diabases or andesites, or augitic diorites. 



The term augite-diorite has already been employed by Zirkel, 1 and 

 its use advocated afresh by Grenville A. J. Cole. 2 The sanction of 

 such a term, although its meaning is evident and not opposed to the 

 observed mineral constitution of these rocks, may however, lead to 

 some present confusion in petrological nomenclature, since the cur- 

 rently accepted definition of a diorite indicates that the constituent 

 minerals are essentially triclinic felspar (labradorite or oligoclase) 

 and hornblende. The important point in naming rocks of this mixed 

 character is to indicate their relationship on either hand to sharply 

 and of course arbitrarily defined rock types when their position is so 

 evenly balanced between two types that the rock cannot be referred 

 to the one type more than to the other, and, to indicate this, I would 

 suggest that the names of the two nearest types should be combined, 

 a preponderating affinity towards the one type or the other being 

 denoted by underlining the name of the dominant type. Thus in the 

 case of those olivine-bearing rocks which approximate on the one 

 hand to diorites and on the other to basalt, we should have 



Diorite. Diorite-Basalt. Diorite Basalt. Diorite-Basalt. Basalt. 



A nomenclature based upon mineralogical terms only is never likely 

 to be adopted, since it is far too cumbrous. Bock-names taken singly 

 fail to indicate transitions in mineral constitution, while a mixture of 

 mineralogical and petrological names at once impairs the needfully, 

 but unnaturally sharp definition of a type. It therefore seems better 

 to employ already well-known words and preserve, so far as possible, 

 the landmarks of classification, than to destroy the meaning of a 

 term by a mineralogical prefix or affix which in conveying a small 

 truth may breed a great misconception. If, for instance, augite or 

 olivine were assumed to be anything more than occasional and 

 subordinate constituents of a diorite, the commonly accepted definition 

 of that rock would become worthless, since the term diorite would 

 then embrace a number of rocks of far greater importance and wider 

 distribution, which have hitherto been distinguished by other names. 

 That a relationship exists between such rocks, and that they 

 graduate one into another, is too well known to need any comment, 



1 " Lehrbuch der Petrographie," vol. ii. p. 7, Bonn, 1866. 



2 " The Igneous Eocks of Stanner," Geol. Mag. Decade III. Vol. III. p. 225 

 (May, 1886). 



