THE PETEOGBAPHY OF SOME QUEENSLAND OIL SHALES. 127 
It is anisotropic, and in the vertical section there are positions of extinction 
parallel and at right angles to the bedding plane, maximum illumination 
occurring at intermediate positions. In the vertical section it also shows a type 
of “cross hatching” or rectangular arrangement of polarisation laminae, the 
laminae appearing when the bedding plane makes an angle of about 27 degrees 
on either side of the vibration direction of one of the nicols. The angle between 
these laminae and the bedding plane has been found to be about 70 degrees. 
As each of the gelosite bodies behaves in a similar manner, they all show 
their polarisation laminae at the one time in the same direction, giving the 
effect of optical continuity. This seems to indicate, as Dulhunty (1939, p. 186) 
has suggested, that the pressure that flattened the translucent » bodies parallel 
to the bedding was responsible for some form of internal strain in the gelosite, 
giving rise to its polarisation phenomena. 
The retinosite is much less abundant than the gelosite. It is orange- 
yellow in colour and quite distinct from the pale yellow of the gelosite. It is 
also less transparent than gelosite, and has a slightly higher relief. In all of 
its other optical properties, however, it is similar to gelosite, and the difference 
between these two bodies thus is presumably biological rather than the result 
of varying conditions of preservation. 
Both the gelosite and the retinosite bodies have been found to be partly 
replaced by chalcedony. The grains of chalcedonic silica usually occur in the 
central part of the bodies and take the form of irregular masses, which are 
elongate in vertical section and fill the spaces that represent the collapsed central 
cavities of the algal colonies. 
The groundmass of the rock is partly made up of small amounts of the 
substance which Dulhunty (1939, p. 187) calls “humosite”. It is deep 
brownish-red in colour only in the very thin marginal area of the section, 
elsewhere appearing quite opaque. It shows no definite habit or internal 
structure, and is distributed through the matrix in such a way that it seems to 
have been moulded round the gelosite and retinosite bodies. Unlike gelosite 
and retinosite it has a high relief and it is isotropic. From the general character 
and appearance of its irregular particles the humosite seems to be a solidified 
humic product of decomposition rather than some specific organic material. 
Most of the matrix consists, however, of mineral matter to which Dulhunty 
has given the general name of “ matrosite. ’ ’ It is homogeneous and opaque, 
and consists of very finely-divided clay together with a few very small crystals 
of pyrites. It forms only a small part of the rock, and in places the skeleton 
of this matrix becomes discontinuous and fragmentary. 
Under a high magnification the gelosite and retinosite bodies show the 
internal biological structures recently described by Dulhunty (1944, p. 30), 
from which he concluded that they were fossil forms of a colonial unicellular 
alga closely related to the living Botryococcus \ braunii . 
