vi William Edmond Logan 269 



how Murchison and Sedgwick cleared up the confusion of 

 the Transition series and created the Devonian, Silurian 

 and Cambrian systems. In Wales certain schists had 

 been detected by Sedgwick below his Cambrian rocks, but 

 they did not greatly interest him, and he never tried to 

 make out their structure and history. Afterwards A. C. 

 Eamsay and his associates claimed these schists as meta- 

 morphosed parts of the Cambrian system. To this day 

 their true position has not been settled further than that 

 they are known to be pre-Cambrian. 



The vast and varied series of rocks, which have now 

 been ascertained to underlie the oldest Cambrian strata, 

 have undergone much scrutiny during the last quarter of 

 a century, and their true nature and sequence are begin- 

 ning to be understood. The first memorable onward step 

 in this investigation was taken in North America by 

 William Edmond Logan (1*798-1875). Many years before 

 his time, the existence of ancient gneisses and schists had 

 been recognized both in the United States and in Canada. 

 At the very beginning of the century, the wide extent of 

 these rocks had been noted by W. Maclure, the father 

 of American geology, who was the first to produce a 

 general geological sketch-map of a large part of the 

 United States. In 1824 and afterwards, Bigsby spent 

 much time among these rocks to the north of Lake 

 Superior. Subsequently the gneisses of the Adirondack 

 Hills were described by Eaton. At the very beginning of 

 his connection with the Geological Survey of Canada in 

 1843, Logan confirmed the observation that the oldest 

 fossiliferous formations of North America lie unconform- 

 ably on a vast series of gneisses and other crystalline 



