vin. RETROSPECT. 99 



At the close of the Poikilitic a?ra we may indulge in a brief 

 retrospect of some of the physical conditions which the successive 

 deposits reveal to us in the sea and on the land. Taking the most 

 general view of the mineral characters of the rocks, and including 

 for this purpose the uppermost or rhsetic beds, we find, in respect 

 of the tints and associated phenomena, several remarkable stages 

 and repetitions. First in regard to the sea : 



BLUE or DARK. Rhsetic beds, rich in organic remains. 



RED. New red and Permian formations of sandstones and shales, with little of 



limestone, and few fossils. [Elsewhere magnesian limestone is not rare in the 



Permian, and shelly limestone (muschelkalk) occurs in the new red.] 

 GREY. Carboniferous limestone formations, rich in various oceanic life. [Towards 



the borders of the sea and in estuaries sandstones, shales, ironstone, and coal.] 

 PURPLE and GREY. Devonian formations ; purple and grey sandstones and shales, 



and limestone ; the grey strata fossiliferous. 

 RED. Old red sandstone ; great mass of sandstones and conglomerates, with small 



and peculiar bands of limestone, containing fishes ; fossils on the whole not 



plentiful. 

 GREY. Silurian strata, composed of limestone, sandstone, and shale ; nearly in 



all parts richly fossiliferous. 

 GREY and PURPLE. Cambrian strata, containing but little of limestone, but much 



dark and grey shale ; toward the lower part purple beds ; the series yields 



in particular dark and grey zones abundance of trilobites. 



These various purple, grey, and red sediments indicate the waste 

 of different shores and surfaces of land. The sea received at suc- 

 cessive times the spoils of different lands by currents arriving in 

 different directions. There was first a pre-Cambrian land, which 

 yielded one set of materials; next a pre-Silurian land, whose 

 mineral constitution was not the same. Then Silurian land ap- 

 peared, followed by Devonian and Carboniferous land. 



Twice in this flow of time came the red deposits which may 

 be called exceptional, and whose origin is not explored. We may 

 indeed suppose the sesquioxide of the Poikilitic series to have been 

 derived from the red haematites of the carboniferous limestone [this 

 haematite is often of Permian age], or, as has been sometimes 

 conjectured, from the old red rocks; but in each of these cases 

 remains the question, ( Whence came originally the red oxide ?' 

 Perhaps, we may answer, from decomposed minerals of volcanic 

 or metamorphic origin ; silicated peroxides altered in an immensity 

 of time by the slow action of the elements. In this point of view 



