148 THE FORMATION OF FLINTS 



traces in the chalk, the microscleres have never yet been 

 observed in the fossil state. That they coexisted with 

 the larger spicules, making up in number for what they 

 lack in size, there can be little doubt, and their universal 

 disappearance is probably due to their minute dimen- 

 sions, which render them an easy prey to solution. 



Thus, then, we have the clearest evidence of the 

 presence at some past time of abundant opal in the form 

 of sponge skeletons in the chalk, and we have equally 

 convincing proof that it has since been removed in 

 solution. But if so, where has it gone, and what has 

 become of it ? Eecalling the association of sponge 

 skeletons with flint nodules, does it not seem possible 

 that the opal w r hich has disappeared in solution has made 

 its reappearance in the form of flint ? The flints which, 

 judging from analogy with the grey ooze of the existing 

 ocean, were not originally present, have subsequently 

 made their appearance ; the sponge skeletons which were 

 originally present have to a large extent vanished. These 

 facts are correlative, and find a single explanation in an 

 affirmative answer to the question just suggested. 



Let us next attempt to discover the nature of the 

 process by which this transformation may have been 

 effected, and for this purpose we wi\l return to the white 

 powder which occurs in the middle of many flints. In 

 outward appearance it closely resembles ordinary chalk, and 

 if we examine it under a microscope w r e shall discover in 

 it abundant foraminifera, coccoliths, and other structures 

 characteristic of this rock, as well as a multitude of 

 sponge spicules and other skeletal remains of siliceous 

 sponges. It is of interest to note with regard to the latter 

 that in many cases they betray obvious signs of solution ; 

 the interior has been eaten away, leaving only a hollow 

 corroded shell behind ; others are solid, but these have 

 suffered a remarkable mineral change; they no longer 



