49 



OCT 11 IMS'; LITHOLOGY. 



Dysgeogenous and Eugeogenoits Strata. — As may be gathered 

 from what has been stated in the chapter devoted to Geology, 

 the rocks of the different periods in North Yorkshire are, for 

 each as a whole, characterised by well - marked lithological 

 peculiarities ; and from this circumstance arises the fact that the 

 tracts which they respectively underlie are found to possess 

 tangibly marked physico-geographical peculiarities. To the 

 question of the differences in mechanical constitution which the 

 various strata present and the results which may be traced as 

 springing from such differences as their primary cause it will 

 therefore be needful to devote a few pages before proceeding 

 further. 



Two principal types of rock in respect of mechanical con- 

 stitution may be traced. Following the nomenclature of the 

 elaborate treatise upon this subject of M. Thurmann * I 

 propose to call them jE'//^^^^^//^?/^ (plentiful-detritus-bearing) and 

 Dysgeogenous (sparing-detritus-yielding), these terms being better 

 calculated than any which have been previously employed which 

 I can call to mind to keep prominently in view what is essential 

 to be remembered. 



The essential differences between the rocks of the two kinds 

 are mainly in respect of their hardness, their [)ower of absorbing 

 and retaining moisture in small masses, their permeability when 

 we consider them as forming extensive strata, and the differences 

 in their power of yielding detritus which result from these 

 characteristics. If we arrange the different kinds of rock in a 

 scale according to their hardness and permeability, beginning 

 with those which possess the smallest capacity for absorption 



* Essai de Phytostatiqiie applique i la chaine du Jura par Jules Thur- 

 mann, Berne 1849, London, Williams and Norgate. See also a pamphlet 

 of my own entitled " An attempt to classify the Flowering I'lants and 

 Ferns of Britain according to their Geognoslic relations," published in 1855. 



JJot. Trans. V.N. U., Vol. 3. E 



