EOBERT HAY — SANDSTONE DIKES. 



53 



The structure of the dike is such as fully to justify the term " wall." There are 

 both vertical and horizontal lines of fracture, the former being at right angles to the 

 line of the wall. Thus it breaks naturally into blocks, which are all rectangular. 

 The material is sandstone. 



The wall is unmistakably a dike. When the exact age of these White river 1 teds 

 is determined, the age of the dike will be known. It does not on either side reach 

 the top of the ravine, and a bluff of much greater elevation a few hundred feet away 

 shows no sign of its presence ; so it may be definitely regarded as having been 

 intruded before the completion of the deposit of the soft clays and marls. One of 

 the evidences of intrusive character lies in the structure of the laminated sheets on 



Figure 20 — Dike number 2. 



cither side of the dike. In these the lamina:' furthest from the dike are more argil- 

 laceous 1 lian those inside, and the inside laminse are decidedly grooved, with verti- 

 cal ridges, and grooves to correspond, on the Bides of the wall itself. The laminated 

 structure on either side is from 2\ to •"> inches thick, and separate laminse vary 

 from one-eighth to five-eighths of an inch in thickness. 



Half a mile in a westerly direction is dike number 2. In general, i1 is similar to 

 number 1, but there are minor differences. It also crosses a ravine, which is nar- 

 rower than the other ; and the dike appears to be the cause of this, as it seems to have 



checked erosion, particularly on the western side. Its exposed length is loo feet ; 



its average thickness is nearly uniform at 13 inches. The vertical laminated 



