35 



rock, to be deposited in circular layers ; or entirely excavating a cylin- 

 drical cavity in the sand, before it had assumed the consistence of rock, 

 and leaving it to be filled subsequently. 



The following sketch is from the surface of the rock, in a field adjoin- 

 ing Somerville village. 



The existence of smaller circular masses in the border of, or entirely 

 within a larger one, without in the least interfering with the stratifica- 

 tion of it, shows that the causes which produced them operated at dis- 

 tinct intervals of time, although their ages appear to be very nearly the 

 same. 



This must, upon the whole, be considered a very interesting problem 

 in Geology. F. B. H. 



N. B. Mr. McAlpine's donation will be found in the additions to 

 the " Historical and Antiquarian Collection." 



