WEAVER: INVERTEBRATE PALEONTOLOGY AND HISTORICAL GEOLOGY 717 



pointed to study them and in 1904 placed the Animikie in the Upper Hiironian 

 and unconforably beneath the Keweenawan. 



In his earlier work along the north side of the St. Lawrence Valley Logan 

 described a very thick series of metamorphosed limestone and schist, which he 

 named the Grenville series. These sediments contain thick beds of graphite, 

 which are considered to have been derived from some very early organic source. 

 Pre-Cambrian rocks are well exposed in the gorge of the Grand Canyon of 

 the Colorado River and were first made known by Powell in 1875. The L^pper 

 pre-Cambrian sandstones, shales, and limestones were described as being 10,000 

 feet thick and were named the Grand Canyon series. They are separated by a 

 profound unconformity from the Cambrian above and also from the older horn- 

 blende and micaceous schists and gneisses below, which later, in 1886, were 

 named the Vishnu schist by Walcott. Pre-Cambrian rocks similar to those in 

 the Grand Canyon have been described from the area of the Little Belt IMoun- 

 tains in Montana in the reports of the Hayden Survey in 1872-1873, by Davis 

 in 1886 from the headwaters of Belt Creek, by Peale in 1893-1897, by Weed and 

 Pirsson in 1896, and by "Weed in 1899 from the Fort Benton and Little Belt 

 Mountains quadrangles. These rocks consist of a lower group of greatly meta- 

 morphosed rocks, separated by a marked unconformity from the upper Algon- 

 kian sediments, which were named the Belt series. The latter are -unconformable 

 beneath the Cambrian. 



The fossil remains of algae, worm burrows, and sponges have been found in 

 the Algonkian rocks but the only evidence of life in the older groups are car- 

 bonaceous slates and graphite. Accordingly, the older rocks have been referred 

 to as Archaeozoic and the younger as Proterozoic, although the United States 

 Geological Survey uses the term Proterozoic for all pre-Cambrian rocks, with 

 the two subdivisions Archaean and Algonkian. 



Recently, Rankama (1948) has shown that the C^'/C^^ ratios in a number of 

 pre-Cambrian carbon-bearing rocks of Finland are similar to the ratios present 

 in many organic substances and not similar to the ratios in inorganic accumula- 

 tions of carbon. It thus appears that the carbon in these rocks was accumulated 

 by organisms. Rankama concludes that the problematical Corycium enigmati- 

 cum Sederholm from the late Archaean of Finland is a real fossil, probably a 

 primitive alga. This method appears to offer much promise for the determina- 

 tion of the remains of organisms in pre-Cambrian sedimentary rocks. 



Lower Paleozoic: The controversy concerning the classification of the lower 

 Paleozoic rocks of Great Britain during the first half of the last century con- 

 tinued until 1879 when Lapworth proposed the term Ordovician system for beds 

 previously called Lower Silurian and Upper Cambrian. Thus the lower Paleo- 

 zoic rocks were classed as three independent systems under the names Cambrian, 

 Ordovician, and Silurian. Investigations carried on by the Geological Survey 

 of Great Britain during the past one hundred years show that the Cambrian 

 rocks of the British Isles consist of more than 12,000 feet of sandstones and 

 shale which have been strongly folded, faulted, and in places partially meta- 

 morphosed. The Ordovician formations are best represented in Wales and west- 

 ern England. 



Important investigations on the lower Paleozoic strata of Bohemia were made 

 by Joachim Barrande and published in twenty-two volumes from 1846 to 1883. 



