376 J. D. Dana — On displacement through intrusion. 



displacement, the largest interval being about a third of an inch. 

 Some of the displaced pieces are partially revolved, and some 

 are tipped out of the plane of the rest of the crystal. The 

 intruded mineral is quartz, a milk-white variety; and its form 

 between the tourmaline fragments is shown by the dotted lines. 



The displacement is evidently not wholly due to any com- 

 pression of the rock, or to movement within it in any direction 

 as a result of pressure or tension — the usual explanation in 

 similar cases (and probable causes of fracturing) ; for the prisms 

 of tourmaline lie, and are displaced, in all directions, and those 

 that are side by side differ greatly in amount and kind of dis- 

 placement as well as in its direction. The displacing agent 

 was silica from an intruding siliceous solution ; and deposition 

 of quartz from the solution must have been continued at the 

 plane of junction with the tourmaline until the displacements 

 were completed. The change of direction and partial rotation 

 of some of the tourmaline sections are evidence of irregular 

 resistance to the displacement in the material of the rock. 



In my note, on the "Decay of Quartzyte," in the number of 

 this Journal for last January (xxix, 57), I refer to the "split 

 and enlarged heads and stems of crinoids" as examples of the 

 separation process. Being aware that crinoidal specimens of 

 this kind had received much attention from Professor A. H. 

 Worthen, Director of the Illinois Geological Survey, I wrote 

 him recently on the subject, and here introduce his valuable 

 letter of October 8th, received in reply. 



"In reply to yours of the 5th inst., I will say that I know of 

 no general report on the geodized fossils of the Keokuk limestone. 

 At the St. Louis meeting of the Amer. Assoc, in 1878, I read a 

 short paper on this subject, and exhibited specimens both of crinoids 

 and shells that had been subjected to the geodizing process. No 

 copy however was furnished for publication. I have seen one 

 specimen of a geodized stem of Barycrinus nearly or quite a foot 

 in diameter and have specimens in my own possession from four 

 to six inches in diameter. Where shells have been geodized 

 their size is not usually increased so much, and I do not now 

 call to mind any specimen where the diameter has been increased 

 more than half its original size. The stems of Barycrinus have 

 a pentaugular central perforation which seems to render them 

 peculiarly susceptible to this process. The stems of large speci- 

 mens of Barycrinus vary from one to two inches in diameter, and 

 increase by the geodizing process from four to six diameters." 



The process here described is of wide geological importance. 

 As the cases show, any intruding mineral may act like the 

 quartz and limonite, in the disrupting, and displacing work. 

 The limit of the width of separation that may thus be occa- 

 sioned remains to be ascertained from further observations. 



