606 CROSS, IDDINGS, PIRSSON, WASHINGTON 



A special and important case of accord between norm and 

 mode is that of the highly feldspathic rocks. It will have been 

 noticed that in the list of standard minerals no reference was 

 made to the plagioclase feldspars intermediate between albite 

 and anorthite (oligoclase, andesine, labradorite, bytownite), nor 

 to the various alkali-feldspars, other than orthoclase and albite, 

 such as microcline and soda-microcline (anorthoclase). This 

 is for the following reasons : 



The feldspars present in a rock are seldom of only one kind, 

 several distinct species being present usually, this being true 

 especially of the plagioclases. 



The individual plagioclase crystals are frequently zonal, so 

 that an exact determination of the relative amounts is a matter 

 of difficulty if not of impossibility. 



When orthoclase, albite and anorthite molecules are present, 

 in rocks of identical chemical composition, the possibilities of 

 combination of these as actual minerals are many. Thus we 

 may have an orthoclase-andesine rock, in which all the albite 

 has crystallized with the anorthite, or an anorthoclase-labradorite 

 rock, where some of the albite has crystallized with the orthoclase. 



We therefore classify rocks, as far as the feldspars are con- 

 cerned, by the calculated molecules of orthoclase, albite and 

 anorthite, irrespective of their actual mineral combinations. The 

 proportions of these will give the mea?i composition of the feld- 

 spars. Any of the possible combinations will then be normative 

 in the sense above, provided that none or only a negligible 

 amount of the feldspathic molecules crystallize as lenads. 



If it should be desired to indicate in the name which combi- 

 nation is actually present, this can be done by the use of the 

 appropriate feldspar name, as described later. Since, however, 

 the albite molecules crystallize with those of anorthite, if these 

 are present in any quantity, it will be necessary to distinguish 

 only the cases in which the albite molecule has crystallized with 

 that of orthoclase, as soda-orthoclase. 



2. Appreciable difference between the norm and mode. 

 — Abnormative minerals (whether standard or not) are in such 



