Apeil 17, 1908] 



SCIENCE 



623 



discussed in a forthcoming memoir, where the 

 correlation of the eastern attenuated Upper 

 Siluric beds will be given. The fauna of the 

 Upper Monroe above the Sylvania sandstone 

 is a remarkable m.ixture of Siluric and 

 Devonic types as recently demonstrated be- 

 fore the Michigan Academy of Sciences, the 

 Chicago meeting of Section E, American As- 

 sociation for the Advancement of Science and 

 the Albuquerque meeting of the Geological 

 Society of America. 



The following classification of the Siluric 

 System of North America is proposed as 

 most expressive of the relationships indicated 

 by the facts now known. 



fj „., . "i r Upper Monroe. 



^™ Middle Monroe (Sylvania sand- 



,, U stone the only known repre- 



Monroan (1 . ■• , 



(900 ft.) I I ^e'ltative). 



ij (_ Lower Monroe. 



A^ Study of the Mineral Constitution of the 

 Chloritic Group Termed Delessite: Alexis 



A. JULIEN. 



In regard to the constitution of minerals 

 and mineral micro-aggregates, as in rocks, it 

 is entirely insufiicient and often misleading to 

 depend upon chemical analyses alone, or upon 

 formulse deduced therefrom, although com- 

 monly this is all that is supplied in the trea- 

 tises. In place of these, a calculation of 

 the actual mineral constitution of the aggre- 

 gate through a recasting of the analyses is 

 required for the needs of the mineralogist or 

 petrographer. All are mixtures. Even the 

 best crystallized mineral has definite mineral 

 impurities. From the well-crystallized form 



to the amorphous compact mass, in which only 

 the microscope can perhaps barely detect the 

 structures and optical behavior of obscure 

 crystalline conditions — all are mineral mix- 

 tures. Eor each group of minerals under such 

 investigations it is found advisable to pre- 

 pare a tabulated scheme comprising all pos- 

 sible mineral constituents, together with their 

 percentage composition. Using this, on the 

 assumption of absolute accuracy of the cer- 

 tainly determined mineral formulse, allowing 

 for the limitations of replacement inherent 

 in each mineral as far as known, and making 

 careful correlation with the ascertained phys- 

 ical and optical characteristics of the identical 

 mineral or micro-aggregate, its mineral con- 

 stitution can be deduced with satisfactory ac- 

 curacy. 



In illustration, this simple method has been 

 applied to a most complex group of micro- 

 aggregates, many of which now pass as 

 definite mineral species — the delessite group. 

 It will suffice here to give one example cover- 

 ing the hitherto accepted mineral " delessite " 

 itself. The analysis is of material from 

 Zwickau by Delesse. 



Per Cent. 



Silica, SiOz 29.45 



Alumina, ALO3 18.25 



Ferric oxide, Fe,03 8.17 



Ferrous oxide, FeO 15.12 



Lime, CaO 0.45 



Magnesia, MgO 15.32 



Water, H^O 12.57 



Total 99.33 



In explanation we are informed by Dana: 

 " Comp.— Perhaps (Groth) H,„(Mg, Fe),- 

 (Al, Fe)jSijO,5." By using, however, the theo- 

 retical percentage composition of each of the 

 minerals stated below, in harmony with the 

 description of " delessite," the figures of this 

 analysis will be found to correspond to a mere 

 mixture with the following mineral constitu- 

 tion : 



Per Cent. 



Prochlorite 72.13 



Halloysite 10.34 



Limonite 9.54 



Colloid silica with water 7.32 



By similar calculations the constitution of 



