POTSDAM PERIOD. 177 



(e.) Mud-cracks (fig. 64) characterize the layers in many places. 



(/.) Wave-marks (§101) occur on some of the layers. 



(g.) Siliceous or cherty concretions, and geodes of quartz crystals, 

 characterize many layers of the Calciferous epoch, both in New 

 York and the West. Even the limestones of Missouri are cherty 

 in some layers. 



The above marks, b to/, are evidences, where they occur, either 

 that the deposits were formed in shallow waters (b, d), or as 

 emerged beaches or flats (d, e,f), or as wind-drifts over fields of 

 sand (c). 



3. Minerals. — The minerals of the Potsdam formations have 

 already been partly enumerated. There are no important beds of 

 ore in New York. Iron-ores occur in Canada. The lead-bearing 

 rocks of Missouri and Arkansas are the magnesian limestones of 

 the Calciferous epoch ; and with the lead-ore (galena) occur also 

 valuable ores of cobalt, and the associated species, pyrites, barytes, 

 calc spar, etc. The copper-mines of the Lake Superior region are 

 in the rocks of this period ; and some remarks upon them will 

 be found on page 195. Quartz crystals in great abundance occur in 

 cavities in the Calciferous rocks of central New York, and fissures 

 are often lined with crystals. Anthracite coal in small pieces is 

 found in some of the Calciferous beds, and fragments are at times 

 imbedded in the crystals of quartz, or lie loose in the cavities that 

 afford the crystals. 



2. European. 



Eocks of the Primordial period have been observed in Great 

 Britain, Scandinavia, Bohemia, and other countries. 



In Great Britain they outcrop in the western half, and are most 

 largely displayed in Shropshire. The rocks in this region are hard 

 siliceous grits and sandstones, and often stand out in rude crags, as 

 at the Stiper Stones ; and they have in places ripple-marks, wave : 

 marks, mud-cracks, and worm-burrows (Scolithus), like the Potsdam 

 rocks of America. Their thickness is from 800 to 1000 feet. They 

 are much inclined, and rest, according to Murchison, "in conform- 

 able apposition upon the upper edges of the Longmynd" rocks, 

 or the Cambrian, as the latter are called (a name first used by 

 Sedgwick). These Cambrian rocks are slates and sandstones, 

 having the estimated thickness of 26,000 feet ; and, although re- 

 garded by Murchison as sub-Silurian, three or four fossils have been 

 detected in them, — viz. : two species of Sea-weed or Corallines (genus 

 Oldhamia) at Bray Head, in Ireland ; and burrows of worms, and a 

 fragment of a Crustacean, in Shropshire. In North Wales there are 



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