Page:Quarterly Journal of the Geological Society of London, vol. 29.djvu/99

1872.] FISHER-CRETACEOUS PHOSPHATIC NODULES. 57 specimens, giving what may be very aptly called a fenestrated pattern to the entire object, appear certainly to have been tubes. The space originally occupied by the walls of the tube has usually been infiltrated by clear amber-coloured phosphate, as has happened to other wider vacuities, as already mentioned. But the central axis of the tube very commonly presents in section a black spot, although it is sometimes vacant, so that the object is pierced with numerous holes.

The minute central cavity of the tubes, filled with the dark mineral, which, as stated, also occupies the minuter cracks, appears to represent the threads of iron oxide which are alone preserved in Mr. Toulmin Smith's dissected specimens, and which led him to adopt his view of the plexus of fibres arranged to "prevent injury from yielding or distortion"*. A dark film of the same mineral covers the exterior of the tube-walls; and I think I can perceive minute dendritic crystals shooting from it outwards into the general mass. The section of the tube therefore, proceeding from the periphery to the centre, presents first a thin dark circle, then a wider ring of clear phosphate, and at the centre a dark spot or else a hole. Sometimes the central spot is absent. A little consideration will show that this structure is in accordance with the course of mineralization, evidenced by a study of the shrinkage-cracks, as already detailed, and points to the walls of the tube and the central hollow having been first of all incrusted by the dark mineral, and subsequently the substance of the tube removed and its place supplied by the infiltration of the phosphate. It is easy to reconcile the various appearances of the tube when the section passes parallel, or nearly parallel, to its axis consistently with the above explanation. Thus, when the section does not remove the outer crust the tube appears dark, or if a thin trace of the crust alone remains it looks like a dark brush; where the upper and lower surfaces are removed we get a clear streak bounded by thin dark lines, sometimes with and sometimes without a dark central line.

The nodes where the tubes intersect present somewhat complex appearances. Their normal character is best seen in fig. 7. The arrangement of the tubes at such a point may be roughly compared to eight rectangular hyperbolas, in three planes at right angles to each other, the branches of the curves coalescing, in sets of four, at a short distance from their common centre. Sometimes the rectangular asymptotes, which form the axis, are themselves a part of the system of tubes at the node, coalescing with the curvilinear tubes, much as a main railway line is connected with its branches at a junction. Sometimes these straight portions are wanting. The peculiar character at the node shown in fig. 4, appears to be explicable in the following way. The four round spots are the spaces included between the tubes which follow the curves and those which follow the axes. If the tubes had been mere lines, these spots would have been triangular spaces with curvilinear hypotenuses. But the great diameter of the main tubes as compared with the nodal area, and the rounding off of the angles, produce the four circular spots,


 * Loc. cit. p. 95.