1871.] 195 [Burbank 



the fact here that the mass of gneiss, the dislocation of which evi- 

 dently formed this little cavern, may still be seen, resting against the 

 dividing wall a few feet below; where it has been exposed by the 

 removal of the limestone. 



I have not been able to ascertain to what depth the limestone ex- 

 tended in the larger excavations. The section (PL I, Fig. 2) does 

 not indicate the relative depth of the original excavations, which are 

 here partly filled with loose rocks and earth. 



It will be seen by this section that if we suppose the strata to have 

 been originally extended upward, and folded or arched over, as actu- 

 ally appears in other parts of the quarry, we should have a fold 

 within a fold, separated by the spaces (C) and (D), which have be- 

 come filled with the limestone; while, as before noticed, portions of 

 the central fold have also been dislocated, forming cavities for the 

 deposition of the limestone. 



That such was the actual structure, and that the limestone was 

 deposited in cavities mostly closed at the top, which have since been 

 uncovered and exposed at the surface by the denuding action of the 

 drift, seems to me a reasonable deduction from the facts observed. 



It is worthy of note that the gneiss of the enclosing walls of the 

 cavities, and that of the dividing bands and the projecting masses 

 nearly enclosed by the limestone, — is all of the same character with 

 that of the surrounding strata not adjacent to the quarry. A speci- 

 men broken from the projecting mass at (c) cannot be distinguished 

 from the ordinary gneiss of this region. In the direction of the 

 strike of the strata also, at a few yards distance from the quarries, 

 the common rock of the region is found with apparently no traces of 

 calcareous matter. In the line along which several of the quarries 

 occur, there are intervals of several miles where no traces of lime- 

 stone have been found, though the ledges are exposed at the surface 

 in numerous places. 



The aggregate length of all the limestone deposits that occur along 

 a line of some twenty-five miles in length, is probably less than one 

 thousand feet. The only other quarry at Chelmsford, which lies 

 nearly in the range with the one sketched, contained a mass of lime- 

 stone filling a cavity of a lenticular form, about sixty feet in length by 

 twenty-five feet in width in the widest part. 



Other quarries in Bolton, Stow and Chelmsford, show a similarity 

 in structure to the one described here. The separation into two or 

 more sections by walls of gneiss, may be seen in at least three of 



