101 



«The result of the pétrographie examination is, that the 

 «sediments which have been examined are found to contain — 

 «though in very varying proportions — detritus both of ordi- 

 «nary granite or gneiss, and of fresh basic igneous rocks. 

 «The examination thus proves that the eruption of the basalts 

 «had commenced before the deposition of the fossiliferous 

 «formations.» 



«The samples examined are: 



"1) The Coeloma-bed. Marly concretion from the greyish- 

 «brown shale (p. 99, Nr. 2). 



"The colour of this concretion is grey; with the naked 

 «eye we see besides the organic remains numerous little flakes 

 «of muscovite. 



»The microscope shows that the principal part is formed 

 «of a dusty brown clayish material; in this material there are 

 «enclosed, besides flakes of muscovite, a number of colourless 

 «grains of sand having a diameter of 0,2 mm. A number of 

 «these are grains of quartz (they were found to be optically 

 «uniaxial with weak positive double refraction); others, optically 

 «biaxial, are polysynthetic anhedra of a plagioclase with large 

 «extinction angles. The grains of plagioclase are perfectly 

 «transparent, and contain no alteration-products. 



«2) The Cyrena-bed. From this bed two different samples 

 «of sandstone were examined. Both are presumably closely 

 «associated with each other; the sample to be mentioned first 

 «consists chiefly of grains of quartz, while the other consists 

 «of grains of relatively high specific gravity. 



«a. Dark brownish-grey calcareous sandstone with nume- 

 «rous and distinctly visible flakes of muscovite. Fossils are 

 «very plentiful in this sandstone. 



«By aid of the microscope I discerned that the grains of 

 «sand belong to the following minerals: 



uQuartz forms by far the greatest number of the grains; 



