OLDER I'EiaBUTlXES OE SCOTLAND. ' 381 



of Labrador. The developDient of the additional series of enclosures 

 causes the mineral to assume a much darker brown tint, while the 

 submetallic reflections become less pronounced. 



It does not, however, appear that the presence of one series of 

 enclosures is absolutely characteristic of the less ferriferous ensta- 

 tites. and that the two or more sets of enclosures are confined 

 to the more highly ferriferous varieties. On the contrary, the same 

 crystal may be altogether free from enclosures in one part, may 

 exhibit one set of enclosures in another part, and thus assume the 

 appearance of bronzite, while at certain points within the crystal a 

 second and a third set of enclosures may appear, and the hyper- 

 sthene structure be produced (see PI. XI. fig. 5). Other things 

 being equal, however, the bronzite-structure is perhaps more likely 

 to be produced in less ferriferous varieties, and the hypersthene 

 structure in the more ferriferous. It is probably too late now to 

 prevent the use of the terms bronzite and hypersthene for varieties 

 differing in comijosition, otherwise it would be well if the names 

 could be applied to distinguish these differences of structure. The 

 parallelism between the varieties of the monoclinic aud rhombic 

 series of pyroxenes is complete. In the one series we have a perfect 

 "mimicry" of the members of the other series : — 



Common Augite corresponds to Perriferous Enstatite. 

 Diallage corresponds to Bronzite. 

 Pseudo-hypersthene corresponds to Hypersthene. 



The slight differences of colour between diallage and bronzite, aud 

 between pseudo-hypersthene and hypersthene, are not sufficiently 

 constant to be relied upon for the discrimination of these minerals ; 

 the only certain means of distinguishing between them being the 

 measure of the cleavage-angle, or the determination of the extinction- 

 angle in the sections for indicating their system of crystallization. 



We have seen that diallage is prone to a further change by 

 the conversion of the augite-substance into green hornblende, and 

 even the separation of calcite along its foliation- planes. By the 

 commencement of this change we get the beautiful green diallage ; 

 while its completion results m the formation of smaragdite, actinolite, 

 or similar amphiboles. 



The enstatites are still more susceptible to changes of the same 

 kind, but the resulting product is altogether different. By taking 

 up water the enstatite substance of the crystal becomes more or 

 less converted into green serpentinous or steatitic substances. 



The greater ease with which the enstatites undergo alterations of 

 this kind than do the augites, is shown by the fact that while the 

 diallage of the Hebridean gabbros and peridotites is almost always 

 unaltered, the enstatites associated with them nearly always exhibit 

 the first symptons of decomposition, and are not unfrequently 

 entirely converted into the hastite-modidiCSitioii. 



The Olivines. — A change analogous to that taking place in the 

 felspars and pyroxenes, is found affecting the olivines when they are 

 traced to great depths from the surface. This change consists in the 



