Murchison s Silurian System. 221 



coal contains, in some parts, thin laminae and veins of white 

 calcareous spar, an appearance which Mr. Murchison ob- 

 serves is of frequent occurrence in the upper secondary and 

 tertiary carbonaceous deposits in various parts of Europe ; 

 an observation of some importance is, that it is this pe- 

 culiar calcareous character of the Singra coal that has 

 rendered such samples as have hitherto been received from 

 that particular bed of Palamow coal inferior. Mr. Mur- 

 chison remarks, that although seldom seen in the best or 

 most ancient coal, yet he has seen this peculiarity in other 

 parts of Shropshire and also in the Dudley field, where 

 the lower beds of new red sandstone begin to pass into 

 the upper beds of the coal measures. 



Mr. Murchison next passes to the consideration of the 

 lower or productive coal and iron field ; and his own words 

 are of so much importance in the following remarks, that 

 we shall quote them as they stand. " It is not easy to 

 give a very precise notion of the structure of this portion 

 of the field without entering into a variety of details fo- 

 reign to my purpose. The labours of Mr. Prestwich however 

 teach us, that the mineral characters of the same strata 

 often change completely within very short distances, beds 

 of sandstone passing horizontally into clay, and clay into 

 sandstone ; that the coal seams wedge out or disappear ; and 

 that sections at places nearly contiguous often present the 

 most marked litholpgical distinctions. These observations, 

 which coincide with my own in various other coal fields, 

 demonstrate the hopelessness of determining the respective 

 ages of such rocks in different localities by shafts and sec- 

 tions, or by a mere comparison of their mineral characters. 

 Even the coal itself constantly tapers away, and disappears 

 amid the shales and sandstones, constituting what are 

 locally termed " symon-faults," the character of which, as 

 distinguished from true faults, is, that the coal wedges itself 

 out and disappears. Mr. Murchison next adduces several 

 sections to show how little persistent parts of beds of 



