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POPULAR SCIENCE REVIEW. 
being interrupted by enclosed minerals ; — and this is pointed out 
as one of its best characteristics under the microscope. Its 
colour is greenish or lemon yellow ; when polarised it shows 
rich Berlin blue and brownish yellow tints. 
Olivine . — The next mineral to be noticed is olivine. It is 
constantly met with in the older trappean rocks, as well as in 
the more recent basalts and lavas. It has been observed that a 
special characteristic of basaltic olivine is that its crystals are 
fragmentary, and frequently the separated parts of individual 
crystals may be recognised, thus most clearly proving the mo- 
tion of the mineral mass in which they are enclosed. Olivine 
crystals are in thin sections, greenish grey; if very thin, 
colourless. It is an easy mineral to distinguish, owing to its 
peculiar granular or undulated-looking surface, and it is also 
frequently fissured, which gives it a veined appearance. It 
polarises with a peculiar opalescent play of rosy and green 
colours. Olivine, more than any other mineral, is inclined to 
decomposition, and this change may be observed to begin along 
its capillary fissures, in which fibrous deposits of oxide of iron 
and of serpentine take place, when its crystals have beautiful 
green reticulations traversing their substance. Sometimes the 
olivine will be completely pseudomorphosed into serpentine, 
and it is worthy of note that long ago, in the earlier editions of 
Sir C. Lyell’s Geological Manual, it was observed that u as 
olivine differs but slightly in its mineral composition from ser- 
pentine, containing even a larger proportion of magnesia than 
serpentine, it had been suggested, with much probability, that 
in the course of ages some basalts, highly charged with olivine, 
may be turned by metamorphic action into serpentine.” 
Olivine is very generally met with as a pseudomorph, the 
irregular patches into which the polariscope shows it to be 
broken up at once showing the distinction between an aggregate 
of mineral matter and a single uniform crystal. Fine hexagonal 
crystals of this mineral are frequent in some of the German 
basalts. 
Leucite . — Leucite is an important constituent of Vesuvian 
and of some other lavas, in which it seems, in part, to replace 
the felspar of other similar lavas. It is also frequently present 
in smaller quantities in many of the igneous rocks. Its micro- 
scopic recognition is easy. It is usually met with in rounded 
grains massed together in bands ; its crystal sections are octo- 
hedral, its angles are often rounded. Very fine large octohe- 
drons are frequently found in the Vesuvian rocks. Under the 
microscope, when the polariscope is used, it presents, especially 
in the smaller grains and crystals, the appearance of a glass, 
but in the larger we may often see a very marked and charac- 
teristic feature— -viz. those interference spectra which have 
