PALEOZOIC TIME — CAMBRIAN. 469 



are always more or less doubtful, the above figures can be considered at the best as only ap- 

 proximations. To the great thickness estimated there is the additional source of doubt 

 referred to on page 451, iinder the Archaean. For if 45,000', the temperature in the 

 bottom beds would have been 1800° F., supposing the increase of temperature downward 

 to have been 1° F. in 25 feet of descent, or only twice as great as now ; and if 35,000', 

 it would have been 1500° F. , high enough for the complete metamorphism of the lower 

 beds in the series. And yet there is no metamorphism. 



The Animikie group, of slates, sandstones, quartzyte, etc., on the north shore of Lake 

 Superior, at the east end of Minnesota, about Grand Portage Bay and beyond, has inter- 

 calations of doleryte (diabase), gabbro, and other rocks, much like those of the Keweenaw 

 formation. Supposed tracks or trails of marine animals, mentioned on page 446, are the 

 only fossils yet found. The Cambrian age of the formation is considered probable by 

 many geologists. The igneous intrusions are regarded as laccolithic by Lawson, and as 

 related in time to those of the Keweenaw formation. 



Eastern Eocky Mountain slope. — The Cambrian beds of the Black Hills are red 

 sandstone and with fossiliferous limestone above, pertaining to the Upper Cambrian. 



In central Texas, the beds of the Llano formation of Walcott are confined to Llano 

 and Burnet counties ; they rest on upturned beds referred to the Algonkian by Walcott 

 (page 447). 



Bocky Mountain region and Pacific slope. — Lower Cambrian beds occur in the Rocky 

 Mountains of British America, on the Vermilion and Kicking Horse passes. At Cotton- 

 wood Canon in Utah, the great section of the Wasatch has at bottom 3000' of quartzyte, 

 and above this 250' of hard shales, affording Lower Cambrian fossils, some of them 

 identical with eastern species ; then succeed Lower Silurian beds, the Upper Cambrian 

 being absent. Above Ophir City, in Oquirrh Mountain, fossils occur in a limestone over 

 sandstone, the whole 2300' thick. In Nevada, according to Walcott, in the Eureka dis- 

 trict, a section of conformable high-dipping beds 7700' thick, contains below (1) 1500' 

 of quartzyte; (2) 3050' limestone, with Lower Cambrian fossils in the lower 500'; (3) 1600' 

 shale, and above this 1200' of limestone, 350' of shale affording Upper Cambrian fossils 

 at bottom. In the Highland Eange, 125 miles south of the last, are 1450' of limestone 

 and shales overlying 350' quartzyte which are Lower Cambrian, and above these, 3000' 

 of massive limestone which are Upper Cambrian. '; 



Other sections occur east of Pioche ; at Silver Peak ; at the south end of the Tim- 

 pahute Range. In Arizona, at the Grand CaiSon of the Colorado, 3000' to 5000' deep, 

 underneath horizontal Carboniferous and Subcarboniferous beds, the lower the " Red 

 Wall Group " of Powell, lie horizontally 700' to 800' of shales and sandstones, the Tonto 

 group of Gilbert, made Upper Cambrian ; the highly tilted beds beneath are referred by 

 Walcott to the Algonkian. In S. E. California, Inyo Co., Lower Cambrian (Wale, 1894). 



For an extended review of the Cambrian of America see Bull. 81, U. 8. G. S. (1892), by 

 C. D. Walcott, to whom the science is indebted for the discovery of the larger part of the 

 facts. 



LIFE. 



The life of the Cambrian, so far as known, was marine. The plants were 

 Algse (seaweed). 



The animals thus far made out from the fossils are all Invertebrates, 

 They include Sponges, Corals, Hydrozoans, Echinoderms, Worms, Brachio- 

 pods, Mollusks of the divisions of Lamellibranchs, Pteropods, Gastropods 

 and Cephalopods;and also, among Arthropods, Trilobites and other Crusta- 

 ceans. All these groups, excepting that of Cephalopods, were represented 

 in the earliest of the three divisions of the era. 



