ARTHROPHYCUS AND D^DALUS. 



207 



When ridges in higher relief are examined from the side the edges 

 of many closely appressed curved elements may be seen bearing traces 

 of the same transverse corrugations as the base and ascending 

 obliquely from it. Sometimes these elements can be traced some 

 distance into the sandstone. In well weathered specimens they are 

 found to be distinct and separable, lunate in section, the under side 

 the convex, and generally bearing the transverse corrugations and 

 longitudinal groove. They usually appear as branches from the upper 

 side of the basal ridge. This ridge, however, is not a one-piece cast, 

 but is formed of the overlapping lower ends of the elements as they 

 thin out successively. In many instances the elements do not lie 

 regularly one above the other, but a little to one side, so that two or 



Fig. 4 — Diagram showing the general mode of formation in Arthrophycus. 



A, clay stratum ; B, layer of sand in which burrowing began ; C, sand deposited while the 

 burrow was occui>ied. In this, the structure is more or less completely obliterated, probably 

 because of the less stable nature of the sand near the surface, a and 1^. first and last positions of 

 the burrow. The curving lines between indicate the edges of the strips, or packings, made as 

 the burrow was shifted. S, transverse section along the line XY, showing the form and 

 arrangement of the packings and their relation to the burrow ; c, ridge on the under surface of a 

 sandy layer, formed when the burrow penetrates an underlying clay stratum. 



more may be at almost the same level, thus producing a thickening. 

 Again, the displacement may be so pronounced toward the upper end 

 as to produce flaring fascicles. These structures are found as 

 frequently in the sandstone as projecting on its under surface. The 

 upper parts are very rarely preserved and then, but poorly. 



Frequently the study of these compound structures is complicated 

 by their cutting through one another. Sometimes the cuttings take 

 place in such a way that the ridges made by the bases appear to 



