J. D. Dana on the Characters distinguishing Kinds of Rocks. 488 
e consequences to lithology of this introduction of the 
term “plagioclase” were unfortunately great. It was made a 
sufficient definition of a rock to say that it consisted of “ plagio- 
clase and hornblende,” “ plagioclase and augite,” and so on; and 
this is now common in recent memoirs on rocks. It was a con- 
rately stated and described, the account might have been satis- 
factory. But, under both diabase and dioryte, the term “ pla- 
ioclase” is used as if sufficiently defined in itself, and under 
loryte it is given with its aggregate signification alone, no 
mention being made of the particular feldspar the dioryte of 
different localities contains. 
If a dioryte happens to be porphyritic, it is at once put into 
*It should be here acknowledged that Rosenbusch’s very valuable work bears 
the title “ Mikroskopische Physiographie der massigen Gesteine” so that it does 
hot claim to cover the subject of the chemical or mineralogical constitution of 
Am. Jour. Sor.—Tuirp Series, Vou. XVI, No. 96.—Dec., 1878. 
28 
