Dr. J. Allan Thomson — BracJtwpod Nomenclature. 371 



3. The Two Semes. 



On comparing the two series of igneous rocks outlined above, it 

 ■will be seen that there is a well-marked similarity in sequence and 

 characters, but this similarity does not extend to details. The 

 Older Series forms two anchi-monomineralic differentiates, one 

 a femic type consisting of predominant pyroxene with subordinate 

 olivine, the other of a felspathic type provided by the small 

 anorthosite of Portsoy. The Younger Series has only a locally 

 developed monomineralic phase, and its ultrabasic member is 

 provided by a predominant olivine type with subordinant pyroxene 

 and plagioclase, intrusion occurring before the production of a non- 

 felspathic femic magma had been possible. So, felspar is common in 

 the picrite of this series, but is never found in the pyroxene-olivine 

 rocks of the Older Series. The basic members of the tw r o series 

 again present the same general types of gabbro, enstatite or 

 hypersthene-gabbro, and diorite, but are different in the relative 

 development of such types, in the predominance of certain species 

 of pyroxene, and in the widespread occurrence of olivine in the 

 Younger Series. The acid members are perfectly similar, both 

 being biotite-microcline granites. The areal relations of the Older 

 Series and of the Younger Series of the Huntly Mass show the same 

 order of magnitude for the different differentiates of each series. 



The distinction between the two series may be based upon the 

 degree of alteration of the rocks, the character of this alteration 

 among the Older Series, the characters of the predominant mineral 

 in the ultrabasic facies, and the character of the pyroxenes in the 

 two orthorhombic -pyroxene - gabbros. In the field, the general 

 greenish appearance due to the alteration into hornblende of the 

 augite of the Older Series and the occurrence sooner or later of 

 cataclastic foliation in them are of great value. Finally, a most 

 important criterion is supplied by their metamorphic effects on the 

 country rocks. The contact rocks produced by the Older Series are 

 foliated by the later regional folding, whereas the effect of the 

 intrusion of the Younger Series is to obliterate this foliation. 



It may be stated, therefore, that in this district there are two 

 petrogenic cycles separated by an epoch of great earth movement. 

 Between Huntly and Portsoy the rocks of these two cycles have 

 risen along almost the same belt of country and present great 

 similarities in their main characteristics. 



In conclusion, the author wishes to express his indebtedness to 

 Dr. John Home, F.B.S., and to Dr. J. S. Flett, F.E.S., for many 

 helpful suggestions. 



V. — Brachiopod Nomenclature : Spirifer and Syringothyris. 



By J. Allan Thomson, M.A., D.Sc, F.G.S., Director of the Dominion 



Museum, Wellington, New Zealand. 



ACCORDING to a strict interpretation of the international rules 

 of zoological nomenclature the generic name Spirifer is wrongly 

 used for the group including Anomites striatus, Martin, and should 

 be restricted to the group including Anomites cuspidatus, Martin, 



