CHAPTER X 



THE CARBONIFEROUS SYSTEM (continued) 



II. WESTPHALIAN SERIES 



A. SUBDIVISIONS AND GENERAL RELATIONS 



As explained on p. 245 the second of the three great series of strata 

 into which the Carboniferous System is now divided has received 

 the name of JVestphalian, and to it belong all the beds which have 

 been classed as " Upper Carboniferous " in the British Isles. These 

 generally occur in conformable succession to the Avonian Series, 

 but they often extend beyond the limits of the older series, and 

 come to rest on Silurian or other more ancient rocks. Moreover, 

 even when they overlie the Avonian there are sometimes evidences 

 of a slight break between the two series, and there is always a 

 change in the species of plants above this horizon. 



In the British Isles the "Coal-Measure" Series is generally 

 underlain by a set of massive sandstones which are known collec- 

 tively as the Millstone grit, but this is a very variable group, 

 being sometimes over 3000 feet thick, and sometimes only 

 represented by 300 or 400 feet of sandstone and shale. In the 

 areas where the Coal-measures are most fully developed they reach 

 a thickness of 6000 to 7000 feet, and have been divided into 

 Lower, Middle, and Upper stages. On this basis, therefore, the 

 \Vestphalian Series is divisible into four stages, and has a maximum 

 thickness of about 10,000 feet. 



A more scientific division of the series is one based on the 

 different assemblages of plants that are found in this great 

 succession of strata, and by a study of these Dr. Kidston has 

 recognised three different assemblages, 1 though the two higher 

 blend into one another through a set of " Transition Beds." The 

 particular differences in these plant-assemblances will be mentioned 



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