170 



tion of thick tar or obsidian. He would recall to the minds 

 of the Society his suggestion a year since, that the speci- 

 men then exhibited might have been taken from a mass 

 extended and filling a fissure, not from the bed itself. He 

 still had his doubts about its being a stratified coal. 



Dr. Jackson explained, that in replying to Prof. Rogers's 

 query, he supposed it to refer to the soft Asphaltic coal. 

 A wavy structure does exist, even in the shales, indicating 

 a process of disturbance during their deposition. The coal 

 is found in the form of a true bed, with masses extruded. 

 Dr. Bacon had distinctly demonstrated the existence of 

 vegetable structure in the coal. The shales are of the 

 same age as the adjoining coal, and are full of bituminous 

 matter, so that they b.urn like coal. 



Prof. Rogers said, that whether the coal were in its ori- 

 ginal bed or extruded from it, there was still evidence as of 

 motion in the solidified mass. When a mass of lava fills a 

 fissure, it hardens with a columnar structure transversely to 

 the fissure ; if on a plain, the columnar structure is at right 

 angles to it. Such, also, is the structure of the Hillsboro' 

 coal. May it not be suspected, therefore, that it may have 

 been in a similar condition to lava, solidifying like lava in a 

 columnar form ? There are appearances also of crushing, 

 in which this structure is broken up, which might be 

 accounted for by the meeting of two opposed sets of joints. 



Dr. Jackson alluded to a suggestion which had been 

 made in another Society, that the Sienite of Nahant is a 

 metamorphic rock. He expressed in the strongest manner 

 his dissent from this opinion. He was followed by Prof. 

 Rogers and Mr. Desor, who both concurred with him in 

 opinion. 



A specimen of Brain coral was presented in the name of 

 Hon. Isaac Davis, of Worcester. The thanks of the So- 

 ciety were voted for the donation. 



