Johnson and Warren — Geology of Rhode Island. 21 



amount perhaps sufficient to again raise the feldspar above 

 its melting point. It has been shown by Day and Allen* in their 

 paper on the " Isomorphism and Thermal Properties of the 

 Feldspars," that a plagioclase crystal of acid composition if 

 raised to a temperature somewhat above its true melting point, 

 persists for a time as a metastable phase without the crystal 

 as a whole undergoing molecular deorientation. The crys- 

 tals in such a condition behave like hyperviscous liquids and 

 are capable of yielding to mechanical deformation. If then 

 the feldspar in the rhodose were in such a condition, the 

 crystallization of the olivine and ore under pressure might 

 reasonably be expected to cause marked changes in the out- 

 lines of the feldspar crystals. Add to these conditions the 

 effect of movements in the mass, even if slight, and we 

 have a very probable explanation of the relations between 

 the feldspar and the groundmass minerals. The indenta- 

 tion and molding of the feldspar outlines by the other 

 minerals, the occurrence of small crystals of olivine and ore 

 within the feldspar substance along boundaries, between 

 crystals, near the margins and along fractures, the bending, 

 deorientation and recementing of parts of the feldspar crys- 

 tals, in short, all the peculiar textural features of the feldspar 

 crystals and their relations to the other minerals as seen in thin 

 section are what might be expected if such a state of affairs 

 as outlined above had obtained. 



One can hardly escape contrasting the conditions of stable 

 equilibrium which obtained in this rock with those existing in 

 the much better understood systems of salts and alloys of the 

 laboratory. The phases found here are, plagioclase, olivine 

 and ore, the latter, as pointed out, existing texturally as a unit 

 although it is made up of three distinct minerals. We have in 

 the rock no textural evidence of the existence of eutectic 

 structures nor of solid solutions between the phases. The 

 ore alone bears a possible analogy to the cases of salts and 

 alloys. In the case of the feldspar . it is perhaps not diffi- 

 cult to understand how it might develop relatively large 

 crystals or groups of crystals in a comparatively mobile mag- 

 ma, such as the molten olivine and ore would make when 

 melted. The feldspar is apparently entirely immiscible in 

 the magma, or the solid phases resulting from the latter's 

 consolidation, by the time it has reached its freezing point. 

 With the ore and olivine it is more difficult to see how they 

 could have separated in the way they have if the separation 



*This Jour. (4), xix, 1905. It is assumed by the writer that the high vis- 

 cosity of the plagioclase At>i An, would be sufficient to cause it to behave in 

 a degree like the more highly acid feldspars. 



