26 II [STORY OF C OH ASSET. 



away, and their cavities have been filled by soil that con- 

 ceals them. In some parts of the woods or pasture lands 

 these black bands of diabase can be traced in straight lines 

 for many hundreds of feet ; they disappear under the grass 

 at the edge of one ledge only to reappear in the next 

 ledge going straight on. In some cases the band of 

 diabase grows narrower as it leads along, and probably it 

 thins down to nothing a few hundred feet farther in that 

 direction. 



In other cases they branch, making smaller dikes that 

 continue to grow narrower. One of this sort is to be seen 

 upon the east end of Beach Island, at the edge of the sea, 

 a few hundred feet north of Cunningham Bridge. 



Besides growing narrower as one follows them horizon- 

 tally, the dikes also become thinner at the surface than 

 they are farther down. If more of them could be seen 

 with a deep vertical exposure this fact would be better ap- 

 preciated. As it is, perhaps the best example for observa- 

 tion is that giant of all the east-west dikes at the notch 

 on Jerusalem Road next to the sea where the spring is. 

 There is a vertical exposure of about twelve feet at this 

 notch in the shore, and the dike is seen to be several feet 

 broader at the bottom than at the top. How much wider 

 it gets to be farther down no one can tell. Hundreds or 

 perhaps thousands of feet farther down, where the lava be- 

 gan to squeeze its way upward through the granite, the 

 crack may have been widened to several times the thick- 

 ness we now see upon the surface. 



It has already been noticed that the sides of these dike 

 walls are not always exactly vertical. 



The first one of the porphyrite dikes at Green Hill 

 Beach slants as much as twenty degrees towards the east. 

 Three of the east-west diabase dikes slant or hade to the 

 north about the same angle. Two of them are at the ends 

 of Pleasant Beach, and one on the east end of Beach 

 Island. 



