166 
MELLO: MICROSCOPICAL STRUCTURE OF ROCKS. 
Diallage, when crystallised, coiTesponds with augite, but 
it is more often found in gTanular aggTegatious, and filling up or 
bordering cavities in the matrix : its colour is green or brownish. In 
sections it shows distinct striatiou paralled to the chief crystallo- 
graphic axis, and a somewhat concentric structure is sometimes denoted 
by little colour rings. Like other minerals, enclosures are frequent in 
it. Diallage is only an altered form of augite. 
Olivine is of frequent occurrence in igneous rocks, both in 
the older as well as in the more recent. Sections of olivine crystals 
under the microscope are greenish grey or colourless, and tlie mineral 
has a peculiar granulated looking surface, which becomes very distinct 
when polarised, having an opalescent appearance. The crystals are 
also frequently fissured. Along these fissures metamorphism is often 
seen, fibrous deposits of oxide of iron and of serpentine are common, 
giving the mineral a green reticulated look. Sometime^; the olivine 
will be completely changed into serpentine, hi some rocks hexagonal 
crj^stals of olivine occur, octagonal as well as unsymmetrical sections 
are also frequently seen, depending upon the angle at which they are 
cut. The angles of the crystals are often rounded. When occurring 
in basalts it has been observed that the olivine crystals are frag- 
mentary, and the separated parts of individual crystals are often 
present, proving the motion of the enclosing mineral mass subsequent 
to their formation. Olivine is very generally met with as a pseudo- 
morph, and the irregular patches into which it is resolved by the 
polariscope shows clearly the distinction between an aggregate and a 
single uniform crystal. 
A common constituent of some of the lavas of Vesuvius, the 
Eifel and of a few other localities, is Leucite, a mineral which seems 
in part to replace the felspar of other similar lavas. It is also present 
in smaller quantities in some few other igneous rocks. It is easily 
recognised under the microscope, occurring either in rounded grains 
grouped together in bands, or in fine octahedral crystal sections. In 
small grains its behaviour is like a glass, but when occurring in larger 
masses it presents when polarised a very marked and characteristic 
appearance, viz. : interference spectra or lamellar polarsiation, which 
has already been described. The leucite crystals between crossed 
