54:8 R. M. Field — Roentgen Ray in Paleontology. 



As we shall see later, we are dealing, in practice, with differ- 

 ence in density as well as difference in atomic weight, the 

 closeness with which mineral particles are bound together and 

 the amount of foreign matter which they (the crystals) occlude 

 during their crystallization having an important effect on the 

 degree of the penetrability of the mass. Few, if any, petrifac- 

 tions have a uniform density throughout. 



For the next experiment the writer selected a specimen of 

 Triarthrus oecki from the Utica shale of Home, N. Y. This 

 well-known species of trilobite is found in this locality as a 

 pseudomorph of pyrite whose iron content causes it to be much 

 more impervious to the ray than the highly carbonaceous shale 

 in which it is embedded. A section of the shale 3 mm in thick- 

 ness was prepared (see fig. 9) placed upon the X-ray plate in 

 the same position as shown in the figure and subjected to the 

 ray. The results obtained are illustrated in figures 10 and 11. 

 Unfortunately it is impossible to get the same amount of 

 detail in the reproduction as in the original skiagraph. In the 

 original skiagraph a close examination shows certain covered 

 portions of the trilobite to be revealed. As shown in the 

 accompanying figure the left antenna and the entire border of 

 the shield are outlined in the skiagraph, 

 while they are not shown in the photo- 

 graph. Fig. 9 also serves to illustrate 

 the effect which the ray has on the 

 shale itself. The black spots represent 

 segregated material, probably pyrite, 

 more impervious to the ray than the 

 bulk of the shale. It will also be no- 

 ticed in the same illustration that the 

 inequalities in surface are faithfully re- 

 produced. This experiment alone does 

 not prove, however, that the X-ray will 

 discover all fossil structures of a sub- 

 surface nature. Organisms which are 

 replaced by the heavy metallic sulphides 

 are quite rare as compared with the 

 number of calcareous and siliceous forms. 

 What is more, it is unusual to find fos- 

 sils where different parts have been 

 replaced by minerals of distinctly differ- 

 ent atomic weights, as, for instance, in 

 the Atrypa reticularis with calcareous valves and pyritic 

 spires, or in certain fossils with calcareous valves and siliceous 

 spires. Also in both these cases investigation of the internal 

 structure is readily permitted by dissolving the cover with 

 hydrochloric acid. Paleontologists have been bothered for 



Diagrammatic sketch 

 made from skiagraph of 

 the specimen of Triar- 

 thrus becki shown in fig. 

 9 of Plate viii, x 2. On 

 comparison with the 

 specimen, it may be seen 

 that the skiagraph brings 

 out the outline of the 

 trilobite and several ap- 

 pendages which are 

 buried in the matrix. 



