292 GENERAL OBSERVATIONS. 



even the blue limestone, which is one of the most constant, undergoes 

 a material change towards Sutton and Coxwold. These variations 

 in the nature of beds increase the difficulty of tracing the connections 

 of the strata. 



11. Some portions of the strata are subordinate to others; and 

 some seams, or inclosed masses, appear to be secretions from the 

 beds that contain them. — Among the rocky substances of this sub- 

 ordinate rank, we may notice the following; flint, in the chalk; chert, 

 in the oolite; the rounded nodules or balls, in the calcareous sand- 

 stone; ironstone nodules, veins^ and seams, in different kinds of shale, 

 and of sandstone ; lias and pyritous septaria and nodules, in the upper 

 shale; the same nodules, and the lias seams, in the alum shale; 

 and blue and green seams, with gypsum veins, in the red marl. 

 All these substances, which do not form separate beds by themselves, 

 but are included in other members of the strata, are to be regarded 

 as subordinate to the beds which contain them ; and most, if not all 

 of them, appear to be secretions from these beds. The process of 

 secretion, however, is by no means of a recent date ; as is the case, in 

 some instances, with the formation of calc spar in crevices of the 

 strata, and selenite in shale ; but must have taken place about the 

 time when the strata were deposited, or before they were fully indura- 

 ted. The appearance of the flat nodules, and imperfect seams, of 

 flint, of ironstone, and even of lias, strongly impresses us with the 

 idea, that these substances were originally diff"used through the mass 

 of the strata to which they respectively belong; and trickling down- 

 wards, were collected into the horizontal seams of the strata, spread- 

 ing themselves according as they could find room to collect or 

 expand, and accommodating their forms to the vacancies, or softer 

 spaces, into which they insinuated themselves. Thus, the nodules 

 and seams of ironstone are most abundant towards the lower part of 

 the shale beds containing them; and where a bed of sandstone closes 



