234 



On the MicRosconc Structure of Flints avd Allied 

 Bodies. 



By M. Hawkins Johnson, F.G.S., &c. 



{Read February 27th, 1874 ) 



The great advance made by geology as a science during the 

 last fifty years is due, not to the acuteness o^ modern ob- 

 servers as compared with their predecessors, but to the arssistance 

 they have derived from the advance of other sciences, upon which, 

 indeed, geology is almost dependent. Many are the aids to in- 

 vestigation which it has thus received, and to none of these is it 

 more indebted than to the microscope, which is almost daily re- 

 vealing new wonders to those who will avail themselves of the 

 assistance it offers. 



I have lately been giving my attention to the curious group of 

 bodies commonly known to geologists as nodules. There are 

 nodules of one sort or another in almost all the sedimentary 

 deposits, from the oldest to the most recent ; and of whatever 

 material they may be composed, they have been almost invariably 

 described as concretions, a term which, as applied to these bodies, 

 can scarcely be considered explanatory. 



To give a list of all that are known would entail my dragging 

 you through a complete course of stratigraphical geology. There 

 are some, however, which are comparatively familiar to us all, such 

 as the Flints and Iron Pyrites of the Chalk, the Septaria of the 

 London and Kimmeridge Clays, the Phosphatic Nodules or 

 Coprolites of the Gault, and the Nodules of Clay Ironstone. 

 These well-known examples are those that I have more particu- 

 larly examined, and of which I wish to speak. 



In my investigation I began by making thin sections, mounting 

 them in and on Canada balsam in a variety of ways, and using 

 both transmitted and reflected light. Of course, I saw what 

 everybody else has seen who has adopted this system, numbers of 

 curious things imbedded in numbers of curious substances ; very 



