Page:Walcott Cambrian Geology and Paleontology II.djvu/233

 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 Dürrberg; 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 1 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 sericiteschist. Owing to the Burgess shale member of the Stephen formation being overlain and underlain by massive limestones it is very frequently metamorphosed and cleaved into schists or soft calcareous or siliceous slates.

With the exception of Marrella splendens and Hymenocaris perfecta, Agnostus, and Microdiscus, the fossils are irregularly distributed 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,