XXVIII. — Observations on the Formation of the Chalk Strata, 

 and on the Structure of the JBelemnite. By Thomas Al- 

 lan, Esq. F. R. S. Edin. &c. 



fRead 2d April 1821 J 



vIF the various rocks of which the Earth's surface is com- 

 posed, there are some which present very satisfactory indica- 

 tions of their origin ; while others, when considered in that re- 

 spect, seem to be placed almost beyond the reach of hypothe- 

 sis. The Granite and Trap rocks, by extending their ramifica- 

 tions among their contiguous neighbours, distinctly avow, not 

 only a subsequent formation, but, in many instances, they present 

 the most unequivocal marks of an igneous origin. The Lime- 

 stones and Sandstones, however, are equally constant in their 

 indications of aquatic deposition, although, at the same time, 

 the assistance of another agency may sometimes be inferred, 

 when their consolidation and position are taken into the ques- 

 tion. Of all the rocks with which I am acquainted, there is 

 none whose formation seems to tax the ingenuity of theorists 

 so severely, as the White Limestone or Chalk, in whatever re- 

 spect we may think fit to consider it. 



3 D 2 This 



