43 



tion of our geognostic horizon. A few days after Prof. Rogers's 

 visit to the quarry, Dr. Jackson, by invitation of Mr. Wainwright, 

 visited it, and made a minute examination of all the geological 

 phenomena which it presents, and obtained specimens of the 

 trilobites through the kindness of Mr. Haywood, and by search 

 at the quarry in company with Mr. Wainwright. Two speci- 

 mens were obtained, one entire, which is 8^ inches long and 4 

 inches wide. 



The other, of which only the head and half the body was 

 obtained is 6 inches wide, and its hood is 7-^ inches across by 

 the base of the head ; hence the length of this specimen must 

 have been 12^ inches at least, which is about the size of the 

 largest specimens of the Paradoxides Tessini discovered in 

 Sweden. The smaller individual has twenty-one articulations, 

 but none in the tail beyond the lateral appendages, and in this 

 respect differs from the P. Tessini, its nearest analogue, which 

 has, according to Brongniart, four faintly marked depressions or 

 folds crossing the tail transversely. They may have been oblit- 

 erated in our specimen by the changes the rock has undergone. 



These Trilobites of Braintree occur in a blue gray argillaceous 

 slate, containing silicate of lime, but no carbonate, and some 

 disseminated iron pyrites. The stratification of the rock, as 

 indicated by its grain and cleavages, dips to the north 50'', and 

 runs east and west. It is but slightly altei'ed by heat in those 

 portions where the trilobites are found, but near the Sienite rocks 

 it is filled with nodules of Epidote, and closely resembles the 

 altered slates of Nahant. There is a small vein of quartz, bear- 

 ing iron pyrites in it, which cuts through the slate strata at right 

 angles. There are also slickensides surfaces on some of the 

 cleavages or joints in the quarry, indicating, as is supposed, the 

 polishing effects of rapid earthquake movements at the period of 

 disturbance of the strata at the time of their disruption by in- 

 truded Sienite. These are all the marks discoverable of meta- 

 morphic action of igneous rocks on their sedimentary strata, 

 though the slate rocks are hemmed in by the Sienite rocks on 

 both sides, and the belt of slate is quite narrow. 



On a hill near the quarry. Dr. Jackson could see the tall 

 steeple of the Baptist Church in Somerset Street, Boston, and 

 on taking its bearings with the compass, it was found to be 



