786 



PRINCIPLES OF STRATIGRAPHY 



across a fossil, a part of which remains with either portion of the 

 spHt mass. 



9. Slatiiicss. This term is appHed to the structure developed by 

 many siliceous and carbonaceous argillutytes of splitting into thin 

 layers or plates parallel to the bedding and with essential regularity 

 of surfaces similar to true slaty cleavage. The Genesee and Black 

 Portage shales of New York, the Black Shale of Ohio and Michigan 

 and others of their kind show this feature well. It is shown in the 

 undisturbed Jurassic lutytes of Wiirttemberg, which are split into 

 slate blocks to all appearance comparable to slates from disturbed 



Fig. 172. Stylolite from Trigonodus limestone ( Muschelkalk) of Frauenthal. 

 Three-quarters natural size. (After Wagner.) (See Fig. 171.) 



regions. Finally a certain slatiness is observed in the Plattenkalke 

 of the Upper Jura in the Solnhofen region, where layers are split 

 thin enough for roofing slate purposes. 



In all these cases the region is undisturbed, and the slaty struc- 

 ture is parallel to the bedding. It is apparently developed as the re- 

 sult of pressure of the overlying rock masses, and, when exposed 

 to erosion, it is developed by the weather in the same way that 

 slaty cleavage is developed by the weather. 



c. Of Complex Origin. 



10. Pressure Sutures and Stylolites.''' In massive limestones 

 are not infrequently found irregular sutures or seams, which pene- 



* For a recent comprehensive survey of the subject together with discussion 

 of all the theories of origin advanced, and reference to literature, see Wagner-24. 



