PRESSURE SUTURES AND STYLOLITES 787 



trate the rock usually in a direction parallel to the bedding planes. 

 These sutures (Drucksutiircii of the Germans) range from a 

 scarcely visible size to projections an inch or more in length. They 

 interlock on opposite sides in a most complicated manner, which 

 has led to their comparison with the sutures of a skull (Vanuxem). 

 Such sutures have been found in limestones of widely varying geo- 

 logical ages and in ditTerent parts of the world. They seem to pass 

 by gradation into the true stylolite structures which generally occur 

 in the same or in similar limestone beds. Stylolites generally occur 

 along a horizontal plane of separation, and consist of flutes and 

 slickensided columns of limestone, varying in length up to 4 inches 

 or more, and in diameter up to two or more inches and projecting al- 

 ternately from the upper and from the lower layer at right angles. 

 At the end of each column is usually a cap of clay, which weathers 

 out in the clifl^, leaving hollow cavities. That these structures are 

 due to pressure is suggested by the fact that the fracture line often 

 passes across recognizable organic remains, the two parts of which 

 are displaced in opposite sides of the fracture plane. The structure 

 thus resembles minute faulting, the flutings on the sides of the col- 

 umn being analogous to the slickensides formed at the fault planes. 

 Stylolites are often mistaken for corals or other organic remains. 

 Not infrequently a shell or other fossil remains caps the stylolite, 

 and determines the outline of the fluted column. 



Ordinary pressure work has, however, not taken place here, 

 for nowhere is there any evidence of deformation of the beds by 

 crowding or compression above the columns, which project from 

 one face of the suture into the hollows of the other. In other 

 words, if the interlocking of the notch-like projections were due to 

 simple compression before or after solidification, then wherever a 

 hollow occurs there should be evidence above and around that hol- 

 low of compression and movement to crowd away the material so as 

 to produce that hollow. 



That the structure was produced after the solidification of the 

 rock is shown by the fact that the surfaces of separation are sharp, 

 that the sides are striated and that all evidence of massive deforma- 

 tion or squeezing is wanting. The most satisfactory theory yet ad- 

 vanced to explain these remarkable structures is that they are the 

 result of unequal solution along sutures or fracture planes. If solu- 

 tion takes place on the concave surfaces of both the upper and lower 

 face of the fracture, the result must be the production of a series of 

 tooth-like projections from both sides of the fissures, wiiich, owing 

 to the pressure of the overlying rock, interpenetrate more and more 

 as room is made by solution. In other words, tlie rock opposite' the 



