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PEOF. J. W. JTJDD ON THE TEETIAEY AKD 



aspect of the augite-crj-stals is completely altered by the development 

 within them of these enclosures. The green and black augite 

 acquires a greyish or brownish grey tint and a submetallic lustre 

 along the planes of foliation. In this condition diallage is exactly 

 analogous to bronzite in the series of rhombic pyroxenes. These 

 characters appear to be intensified by weathering action, which seems 

 to act with great facility along the planes of foliation, giving rise to 

 the formation of laminee of calcite and other secondary minerals, 

 as was pointed out both by Tschermak and Boricky. Under these 

 circumstances the green variety of diallage has its origin in a partial 

 conversion of the augite-substance into hornblende, with other 

 accompanying changes. 



The development of two or three sets of enclosures in mutually 

 intersecting planes causes the crystals of augite to acquire the deep- 

 brown tint and the bronzy lastre of common hypersthene (paulite). 

 In this condition the mineral has received from Dana the name of 

 " pseudo-hypersthene." In its microscopic characters, no less than in 

 its colour and lustre, it exhibits such a striking resemblance to the 

 altered forms of the rhombic ferriferous enstatites (the substance to 

 which the name of hypersthene was originally applied) that we 

 may cite it as a very remarkable example of mimicry in the mineral 

 kingdom. 



The enstatites, and especially their more ferriferous varieties, 

 exhibit the same development of enclosures along certain planes 

 which is found among the augites. Sometimes one such set of 

 planes is developed, and the result is a mineral identical in character 

 with the bronzites and bastites ; at other times two or more sets 

 of such enclosures are developed along mutually intersecting planes, 

 resulting in an appearance like that of the typical Labrador hyper- 

 sthene or paulite. As already pointed out, such a variety was 

 certainly collected by Macculloch in the Cuchullin Hills of Skye, and 

 the same mineral was probably analyzed by Muir. 



In the rock of Coruiskh the very highly ferriferous enstatite (ambly- 

 stegite) occasionally occurs in a perfectly unaltered state. It then 

 appears as a mineral of a rich brown colour, which, in thin sections, 

 shows the striking dichroism akeady referred to, the colours changing 

 from a rich garnet-red to blue-green. In other crystals, however, 

 the commencement of change is exhibited by the development of 

 enclosures along planes parallel to the brachypinacoid (see PL XI. 

 fig. 9). The ferriferous enstatites having one set of interpo- 

 sitions developed within them, exhibit the submetallic reflections 

 and the striated appearance under the microscope so characteristic of 

 diallage. It is not surprising therefore that the foliated enstatite 

 (bronzite) and the foliated augite (diallage) have been so frequently 

 mistaken for one another. 



In many cases a second, third, and even a fourth set of enclosures 

 are seen to be developed within the enstatite crystals, in planes 

 parallel to the macropinacoid, and the prismatic faces (see PL XI. 

 fig. 8). We thus get the structure produced which is so well 

 known as being characteristic of the original hypersthene (paulite) 



