\ 



NO. 6 MIDDLE CAMBRIAN BRANCHIOPODA, ETC. I5I 



4. Kata-biotite-orthoclase gneiss. Corundum-bearing. Waldheim, Saxony. 

 Quoted from Grubenman, " Die Kristallinem Schiefer," 2nd edition, 1910, 

 p. 158. 



5. Range of composition of commercial slate of aqueous sedimentary origin 

 according to Dale, U. S. Geological Survey Bulletin 275, p. 36. 



The analysis shows a remarkable similarity to analysis 2, which is of the 

 mineral sericite from Diirrberg ; after deducting the calcite and pyrite from 

 the slate analysis the similarity is still more striking. Analysis 3, which 

 represents the Mansfield slate of Lower Huronian age from the Crystal 

 Falls District, Michigan, is somewhat higher in silica and lower in aluminum 

 but is otherwise very similar. Analysis 4 represents a kata-biotite-orthoclase 

 gneiss, corundum-bearing, from Saxony, and differs from analysis i chiefly 

 in its lower water content and in the relation between the soda and the potash. 

 The fifth column gives the range of composition of commercial slates of 

 aqueous sedimentary origin as given by Dale. The slate from British 

 Columbia is outside of these limits in many respects; the silica is a 

 little lower, the aluminum is high, the soda low, and the potash high. In 

 general, this rock, as compared with other slates, phyllites, and related 

 schists, is noteworthy for its low content in silica, its high aluminum and 

 potash, and its poverty in all other oxides except water. The excess of 

 potash over soda is especially remarkable. 



The composition of the slate and its microscopic texture show that it was 

 derived from a very fine, highly aluminous sediment, whose material must 

 have consisted of the very finest suspended matter which had been leached 

 unusually free from iron, magnesia, lime, etc., and which consisted largely of 

 kaolinite and quartz. 



It is interesting to note that Analysis 2 of the sericite is so similar 

 to the Burgess shale, owing to the fact that where the Burgess shale 

 is compressed and metamorphosed at the western end of the Burgess 

 Pass beneath Mount Burgess it is to all appearances a sericite- 

 schist. Owing to the Burgess shale member of the Stephen for- 

 mation being overlain and underlain by massive limestones it is 

 very frequently metamorphosed and cleaved into schists or soft 

 calcareous or siliceous slates. 



MODE OF OCCURRENCE 



With the exception of Marrella splendens and Hymenocaris per- 

 fected, Agnostus, and Microdisciis, the fossils are irregularly dis- 

 tributed and of relatively rare occurrence. They are pressed flat 

 even in layers where there are no visible traces of lamination of 

 the rock. 



For convenience of reference I shall call the lower portion of the 

 Burgess shale,^ in which so many beautifully preserved fossils occur, 



^Smithsonian Misc. Coll., Vol. 57, No. 3, 1911, p. 5i- 



