544 R. M. Field — Roentgen Ray in Paleontology. 



by Dr. W. H. Bragg of the University of Leeds, to the dis- 

 covery of the molecular structure of crystals. Such investiga- 

 tions, although of great importance in themselves, are of little 

 service in solving the problem at hand. Bragg deals with crys- 

 tallized, not composite, mineral structures. Unfortunately it 

 was not until his own experiments had been performed that the 

 writer became aware of the work of Bruhl, Doelter and Branco, 

 the latter having made somewhat extensive experiments as late as 

 1906. Branco's results are particularly interesting and well 

 worth attention. The writer believes, however, that his paper 

 has been read by few, if any, paleontologists in this country, and 

 it is largely for this reason that the following communication 

 is presented. 



In 1896 Bruhl* had asked the question : " Can the X-ray be 

 projected through the sedimentary rocks, and if so, what will 

 be the degree of the intensity of the penetration in the differ- 

 ent clastic rocks"? His experiments showed that " mineral- 

 ized fossils wholly or half buried in the rock might be dis- 

 covered." 



C. Doelterf experimenting in 1896 arrived at the general 

 conclusion that certain minerals were more impervious to the 

 ray than others. He states that : — 



1. A comparison can not be made between the "durchlassig- 

 keit" (degree of penetration) and the thickness (under 1 centi- 

 meter). 



2. A comparison can not be made between the chemical 

 composition and the degree of penetration. 



3. Twinning does not affect the degree of penetration. 

 Doelter experimented with a large group of minerals. The 



following table is compiled from a few of his examples : 



Substance Thickness Degree of penetration 



Amber _. l'l mra Easilv penetrated 



Jet 1-2 " " " 



Graphite 2*9 " " 



Kaolin. _. 2*2 " " 



Opal 2-2 Penetrated 



Quartz 2'6 " 



Plagioclase... 2-6-2'7 " 



Gypsum 2*2 Penetrated with difficulty 



Chalk 2-7 



Aragonite 2-9 " " " 



Marcasite 5*3 Impenetrable 



*'• Uber Ver wen dung von Eoentgenschen X-strahlen zu paleontologisch- 

 diagnostischen Zwecken, " Verhandl. Berlin. Physiol. Gesellsch., Archiv 

 fur Anat. und Physiol, p. 547, 1896. 



f ' l Verhalten der Mineralien zu den Eoentgenschen X-strahlen," Neues 

 Jahrb. Min., II. pp. 87-106, 1896. 



