730 



PRINCIPLES OF STRATIGRAPHY 



chic sea. These basal sands indicate by their purity a distant source 

 and long transportation, and the time interval during the Triassic 

 and early Jurassic periods was ample to make possible an extended 

 accumulation of widespread river and eolian sands derived in large 

 part from the crystallines of the Canadian and western uplands, 

 added to no doubt by contributions from uncovered Palaeozoic and 

 older sediments. A striking case of change in lithic character with 

 progress of transgression is seen in the Cretacic series of south- 

 east England and in that of northeast Ireland and the west of 

 Scotland (Mull and JMorvern). (Fig. 146.) In both cases the series 

 begins with basal conglomerates, followed by sands, clays, and 



Fig. 146. Diagram showing overlap and change in lithic character of the 

 Cretacic formations of England and Ireland, from southeast to 

 northwest. 



greensands. This is followed by (2) Glauconite sands and marls, 

 then by (3) marls and Greensand chalk, passing into glauconitic 

 and argillaceous chalk, and, finally, by (4) pure white chalk. In 

 England the basal series ( i ) rests on the Weald clays, and is of 

 Aptien age. while in Ireland it rests on Lias and is of Cenomanien 

 age. In Scotland it also is of Cenomanien age. The next lithic 

 series (No. 2) is of Albien (Gault) age in England, but of Turonien 

 age in Ireland. The succeeding marls and Greensand chalk (No. 3) 

 are of Cenomanien age in England, but of lower Senonien age in 

 Ireland. Finally, the white chalk of England begins in the Turo- 

 nien, but in Ireland and Scotland it occurs only in the upper Seno- 

 nien. This illustrates not only the progressive overlap of the forma- 



