No. 2.] DANA — SOME POINTS IN LITHOLOGY. 83 



clase and augite, " and that the feldspars, oligoclase, labradorite 

 and anorthite have been observed in them. Dioryte is defined 

 as a " family " of older rocks consisting essentially of " plagioclase 

 and hornbleode." Had the different kinds of rocks embraced 

 in these families been separately stated and described, the account 

 might have been satisfactory. But, under both diabase and 

 dioryte, the term ^' plagioclase " is used as if sufficiently defined 

 in itself, and under dioryte it is given with its aggregate signifi- 

 cation alone, no meotion being made of the particular feldspar 

 the dioryte of difi"erent localities contains. * 



If a dioryte happens to be porphyritic, it is at once put into 

 the grand division of dioryte porphyry, when the only distinction 

 may be that the feldspar is in defined crystals, the chemical and 

 mincralogical constitution being identical. Bnt if the feldspar 

 of one dioryte contains twenty per cent, of silica more than another 

 and no soda at all, it is still all dioryte. 



In geology , it is essential to a thorough study of the questions 

 it has before it that the kinds of feldspars sliould not be massed 

 under a common name, and that in every case the investigation 

 should be considered unfinished until not merely the amount of 

 silica in the rock is accurately ascertained, bnt also the particular 

 species of feldspar is correctly and fully determined, however great 

 the labor required to reach a conclusion. The use of the term 

 plagioclase in such a case is an acknowledgment of incomplete 

 work, and should be so treated. 



But the objection to the use of the term *' plagioclase '' is still 

 stronger than has been stated. It now includes not only the soda- 

 lime feldspars from anorthite to albite inclusive, but also part of 

 potash- feldsTpar. The establishment on an unquestionable basis, 

 of Breithaupt's microdine by Des Cloizeaux, and his further 

 observations that this triclinic potash-feldspar is a very common 

 mineral, much of what was supposed to be orthoclase belonging 

 to it, has extended the range of "plagioclase," until it is now 

 almost an equivalent of the general term feldspar, so that ^^ plagio. 

 close 'dnd hornblende " has, as to chemical constitution, the same 

 signification now with feldspar and hornblende. 



* It should be here acknowledged that Roscnbnsch's very valuable 

 work bears the title " Mikroskopische Physiographic der massigen 

 Gesteine," so that it does not claim to cover the subject of the chemi- 

 cal or mineralogical constitution of rocks. 



