88 



ON THE MINERALOGY OF THE 



The first has a basis of coarse slate-clay, with 

 mica and iron-pyrites, and contains numerous irre- 

 gular rounded masses of compact limestone. The 

 limestone is of a dark bluish -grey colour, with a 

 fine splintery fracture, destitute of lustre, and ha- 

 ving a light-coloured dull streak. These pieces are 

 of no determinate shape, and pass, by imperceptible 

 degrees, into the basis in which they are imbedded. 

 Hence these masses, and the basis, are to be consi- 

 dered as of contemporaneous formation. 



Perhaps this test of contemporaneously formed 

 masses, is not much to be depended upon. The 

 petrifactions of shells which occur in the older rocks, 

 are often so intimately united with the matter in 

 which fhey are imbedded, that they appear to pass 

 into it by insensible degrees, it being impossible to 

 detect the line of separation. Yet, in these cases, 

 we do not consider them as of contemporaneous for- 

 mation ; but conclude, that some action has taken 

 place in the matter of the bed, subsequent to its de- 

 position, which has exerted its influence on the sub- 

 stance of the imbedded shells. Examples of such 

 changes may frequently be met with in floetz lime- 

 stones ; and they, perhaps, occur in other rocks con- 

 taining fragments ; although in these last, we have 

 not the means of determining the original form 

 of the imbedded masses ; and, consequently, can- 

 not trace the extent of the changes which they 

 have experienced. 



The second consists of a basis of granular quartz, 

 with irregularly shaped pieces of bluish-black clay. 



