PERTHITIC FELDSPARS. 151 



tion lends support to the idea of a eutectie composition. It will be 

 evident, however, from what has just been said that the occurrence in 

 a rock of richly perthitic feldspars is by no means an indication that 

 such intergrowths are necessarily of eutectie composition, or even 

 very near it, as appears to have been held by some petrographers. 



Mixtures of a composition like that represented by the line ii may 

 be supposed to result vmder certain conrHtions in the so-called anti- 

 perthite intergrowths, while a mixture of composition v, well to the 

 left of the diagram, would yield finally a mixed-crystal of the albite 

 type of the same composition as that with which we started. 



From what has been said above it will be clear that rapid cooling, 

 accompanied probably by undercooling, is believed to be primarily 

 responsible for the development of anorthoclase, cryptoperthite and 

 the perthitic intergrowths generally, where the latter are relatively 

 rich — above something like 28 per cent Ab. Inasmuch as the rate of 

 cooling is dependent on the geologic position in which a magma con- 

 solidates, it follows that the presence of such feldspars are an indication 

 that the rocks containing them have consolidated relatively near or 

 at the surface. If this is correct it is a point of some importance. 

 The anorthoclase phenocrysts of many of our porphyritic rocks bear 

 out this contention, and geologic evidence is by no means lacking that 

 many occurrences of rocks characterized by the presence of richly 

 perthitic or cryptoperthitic feldspars have reached relatively high 

 levels in the earth's crust, for example, the alkaline rocks of the Chris- 

 tiania region and the microperthitic rocks of eastern Massachusetts, 

 particularly of Quincy and the Blue Hills, not to mention many other 

 examples that come to mind. 



We also believe that microcline is the stable phase of the potassic 

 feldspar. The tendency to invert to microcline, like the intimately 

 associated tendency of the two mixed-crystals to unmix, is probably 

 not a strong one, and may often not find conditions favorable to its 

 accomplishment. The change may be suspended indefinitely and 

 even not occur at all, or not until the necessary energy is supplied by 

 long continued heat and pressure such as might result from meta- 

 morphic processes. That microcline is of much more common occur- 

 rence than is generally supposed, the writer firmly believes. It is 

 probably true that many students of rocks when they do not see 

 the characteristic "grating" structure in a potassic feldspar call it 

 orthoclase, overlooking the fact, if they were ever aware of it, that 

 microcline does not always, in fact rather frequently, does not show 

 this structure, that it may show only the albite twinning, or no twin- 



