215 



here averages gentle to the north. The cleavage is less perfect 

 in this direction. 



] This little bay opening to the south-west is unquestionably 

 on the axis of a synclinal, and the slates on either side have 

 the contortions belonging to flexible beds in this position. 

 The structure is typical and instructive. Quarantine Rocks 

 south-west of Rainsford Island form four small islands lying 

 in a north-south line, all but one of which, the most north- 

 erly, are entirely submerged at high tide. This principal rock 

 lies farther north than represented on the map, and, following 

 the line of strike of the slates, is about opposite the middle of 

 the crescent-shaped bay on Rainsford Island ; i. e., it is on the 

 very axis of the synclinal. And here, accordingly, we fail to 

 observe any prevailing dip ; but the beds are most wonderfully 

 contorted and folded ; presenting anticlines and synclines of 

 every variety, — large and small, open and closely folded, nor- 

 mal, inverted, and faulted. But few of the contortions are very 

 small ; they are mostly to be measured by feet rather than inches. 

 This structure is unparalleled among our Primordial strata, and 

 certainly could only be developed among the upper beds in a syn- 

 clinal fold. That these are the same beds as those worked in 

 the Sachem's Creek quarries will hardly be questioned, notwith- 

 standing the seemingly great difference in structure ; but this is 

 probably synclinal in each case, a closed fold on the main land 

 apparently changing to an open, corrugated trough farther 

 east. 



The other Quarantine Rocks are composed of the same 

 grayish-black slate, but the structure seems to be more like 

 that on the south side of Rainsford Island, i. e., with a pre- 

 vailing northerly dip ; the exposures, however, are very poor. 

 Sunken Ledge, nearly a mile south of Quarantine Rocks, 

 consists of loose materials ; one large, angular mass, how- 

 ever, appears to be in situ, or nearly so. It is a brownish slate 

 holding an occasional pebble, and evidently marking a passage 

 between slate and conglomerate. This is on about the same 

 line of strike as Hangman's Island. 



