84 WYNNE : GEOLOGY OF KCTCH. [PART I. 



frequently cleanly washed, coarse grained, and obliquely laminated, beds 

 would seem to indicate shallow agitated water as a condition of their 

 accumulation. This being the case, the small amount of alteration requisite 

 suggests the probability of the occurrence of estuaries or back-waters 

 or fresh-water lakes here and there during the deposition of these rocks. 

 Subsequent to their deposition these upper Jurassic rocks suffered au 

 apparently unequal amount of denudation, they at the time, in some cases 

 certainly, having formed land. A great change then took place, and the 

 Jurassic formation, whether as land or under water, became covered locally 

 by earthy and sandy deposits largely composed of 

 igneous materials, and generally by volcanic flows ; 

 among the earliest of these some small fresh-water lakes existed, 

 wherein fish and molluscs lived. The sources of these volcanic flows 

 are not accurately known ; but it is most highly probable that they 

 issued from the numerous fissures now occupied by traps in the Jurassic 

 rocks. Some doubt may exist as to whether these are sufiiciently numer- 

 ous for all the overlying trap rocks to have thus found vent ; it should 

 however be remembered that such sources may be at least as numer- 

 ous in the area beneath the traps themselves as elsewhere, though 

 probably frequently overlaid and entirely concealed by superincumbent 

 masses of trap, or, by reason of the similarity of the rock, not making 

 themselves manifest at the present surface of the ground. 



The question of these traps having been mainly subaqueous has 

 been already referred to ; this view appearing to me the most probable, 

 from the absence of the cones and craters which generally distinguish 

 terrestrial volcanic regions. 



At the close of the trappean period some volcanic flows of widely 

 different aspect from the rest supervened, sources 



Sub-nummulitio group. 



for which appear to be strongly indicated by the 

 occurrence of similar rock in dykes through the Jurassic formation, either 

 occupying fissures by themselves or sometimes lying along the sides of other 

 ( 84 ) 



