122 The Petrography axd Genesis of Sediments 



samples. The importance of considering the glauconite separately is, 

 however, evident, since in many of the samples it has been formed in place 

 and not brought in like the rest of the material. 



Except for separating the glauconite the electro-magnet plays no 

 inherent part in the analysis of the samples. It has been used merely to 

 segregate different minerals in order to facilitate the study of them. The 

 magnetic permeability of different minerals is distinct, so that, by intro- 

 ducing various resistances in the circuit of the magnet, they can be segre- 

 gated. Thoulet has for his magnet a table showing the current that will 

 attract each mineral, hut this varies so with the particular constitution, 

 and doubtless also with the amount of decomposition of the mineral, that 

 it affords only an approximate indication in practice. It was found most 

 practical to try different strengths of current and examine the product 

 with the hand lens, until a satisfactory separation was obtained. One of 

 the most refractory minerals in both the gravity and magnetic separation 

 is mica. While it tends to accumulate in certain portions, the segregation 

 is always far from perfect, and, moreover, in transferring it there are 

 always losses said to be due to static electric charges which cause it to 

 adhere to the surfaces with which it comes in contact. This very static 

 electric property can be used to separate it from other minerals, but 

 this procedure has not been applied in the present study. 



While the method thus described includes all the steps employed in a 

 complete analysis it appeared, when the results began to accumulate, that 

 some of the separations could not yield information of any value in certain 

 sediments, or at least that more results of importance could be accumu- 

 lated by not making each analysis so systematically complete; hence in a 

 few of the later ones some of the steps are omitted. 



The quantitative results of the mechanical analyses are represented in 

 the diagrammatic form (pp. 169, 170) so effectively used by Mohr in the 

 •papers referred to above. 1 The construction of these diagrams is very 



1 While Mohr devised these diagrams quite independently, exactly the same 

 type of diagram, differing only in scale, was used at an earlier date by J. A. 

 Udden, " The mechanical composition of wind deposits." Augustana Library 

 Publ. No. 1, 1898, 69 pp. 



