EIvISHA MlTCHEI.Iv SCIENTIFIC SOCIETY. 57 



the most general concretionary form, we should still 

 have no difficulty in deciding- that they do not belong- 

 to the last class, since the interior is a compact mass 

 of semi-crystalline quartz, often showing- the layers in 

 which it was laid down by the ordinary process of 

 deposition. (Fig-. 14.) From what has been said of 

 their distribution (p. 54), they could not belong- to the 

 second class. The chalcedonic envelope is distinctly 

 concretionary, and reg-arding- the palaeotrochis as a 

 nucleus, they can belong- to the first class. But the 

 palaeotrochis itself is no ordinary concretion, which 

 Marsh admits and tries to find some analog-y between 

 it and cone-in-cone structure. 



(2) Is it stylolites or cone-in-cone? "Stylolites are 

 cylindrical or columnar bodies varying- in leng-th up to 

 more than four, and in diameter up to two or more 

 inches. The sides are long-itudinally striated or 

 g-rooved. Kach column usually with a conical or rounded 

 cap of clay, beneath which a shell or other organism 

 may frequently be detected, is placed at right angles 

 to the bedding- of the limestones, or calcareous shales 

 through which it passes, and consists of the same 

 material. This structure has been referred by Pro- 

 fessor Marsh to the difference between the resistence 

 offered by the column under the shell, and b3^ the sur- 

 rounding matrix to superincumbent pressure. The 

 striated surface in this view is a case of ' slicken- 

 slides. '"^ 



It is true that the palseotrochis shows sig-ns of pres- 

 sure, but, as already pointed out (p. 55), the pressure had 

 a tendency to deform the structure and obliterate the 

 grooves or striae instead of forming or constructing 

 them, while the layers of deposit of which the forms 



1. Text Book of Geolog-y. A Geikie. 3d. Edition, 1893, p. 316. 



