1872.] FISHER CRETACEOtTS PHOSPHATIC NODULES. 57 



specimens, giving what may be very aptly called a fenestrated pat- 

 tern 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 peri- 

 phery 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 mineral- 

 ization, 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 subse- 

 quently 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 rectan- 

 gular 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. 



