A GEOLOGICAL HISTORY. 63 



and sod, and is sloped in, making it funnel shaped ; 

 the bottom, is some twenty feet below the level 

 of the ocean, and every rolling wave dashes in 

 and throws its spray to the very top. How this 

 opening in the rock was formed is somewhat 

 problematic. The place in former times, might 

 have been filled with softer materials^ which were 

 washed out by the waves of the ocean, continually 

 dashing in — and yet no traces of any decomposable 

 rock remain. The rock could not have been parted 

 by any convulsion of nature, for the cavity is nearly 

 as wide in its further end as at the mouth, and its 

 sides are perpendicular ; resembling an alley be- 

 tween two high buildings, with its back end 

 closed up. 



5th Black Shale. — (See plate 6, jig. 5.) This is 

 a true coal shale, and commences at what are 

 called " the Blue rocks," at the north end of the 

 town, on the shore, below Fort Green. It runs 

 interruptedly, all along the Island to the Ports- 

 mouth Coal Mines, which are about eight miles 

 north of the town. In this shale is seen impres- 

 sions of Lepidodendrons, Sagillaria and other fossil 

 plants. 



6th Rhode Island Coal. — (See plate Q,fig- 6.) 

 This is an Anthracite of a lead blue colour, which 

 is caused, by the diffusion of Plumbago, through 



