pe rca sateen of Rocks and Minerals. 101 
particles thickly scattered through the matrix are crowded together 
_ round the sides of the crystals, having been forced outwards as the 
latter increased in size ; this clearly indicates that during the for- 
mation of the crystals, the matrix was in a viscid, but not in a 
jluid state, for had the particles been quite free to move, there would 
have been no crowding. 
In a section of basalt from the Rhine, the olivine is in its usual 
fractured condition, and some of the larger cracks have been filled 
up with the fine crystallized matrix in which they are imbedded ;— 
there is no crowding of the particles; in this case, therefore, the 
olivine was not only crystallized, but fractured, before the consolida- 
tion of the mass. In another section of basalt the crystals of 
augite and olivine are somewhat rounded, and the cracks filled up, 
so that they probably existed as crystals or grains before the ejection 
of the lava. 
One of the most important aids in the examination of rocks and 
minerals is afforded by polarized ight. In many cases it enables 
us at once to discriminate between different minerals, and not un- 
frequently affords clear evidence of changes which have taken place 
subsequently to the consolidation of the substance under examina- 
tion. When a thin section of a crystal is placed on the stage of the 
microscope, and a beam of polarized light is passed through it, the 
beam is depolarized, and generally exhibits colours due to inter- 
ference ; the intensity of colour varies according to the direction in 
which the crystal is cut, and consequently, in examining a section 
of rock, the various sections of any one mineral do not always give 
the same result ; but as the crystals of igneous rocks lie at all angles, 
it ig nearly always easy to obtain some which, being cut at an in- 
clination to the optic axis, exhibit different degrees of intensity of 
action ; therefore minerals which vary much from each other in this 
respect may be easily distinguished. 
A most important point to be noted is, that the depolarizing 
action of a crystal is uniform over the whole surface of its section, 
if it consists of one simple crystalline structure ; when, however, 
the light appears to break up into detached parts, each of which 
changes independently as the analyzer is rotated, we know that it is 
made up of a number of separate crystalline portions, either inde- 
pendent of each other, or sometimes related as twins.* 
A knowledge of these facts enabled me to detect the presence of 
olivine and its pseudomorphs in the Rowley rock, as described in a 
former paper, and also in the ‘ Geological Magazine,’ vol. vi., p. 115. 
A pseudomorph is a mineral possessing a crystalline form, which 
does not belong to the substance of which it is composed ; it is an 
altered mineral, or, in other words, an aggregate of mineral matter, 
* Sorby “On the Examination of Rocks and Minerals,” in Dr. Beale’s work 
on the Microscope. 
