WESTERX, CEXTRAL, AXD EASTERN SECTIONS 255 



metamorphosed strata thrown into anticlines and synclines whose axial 

 planes display some diversity of dip, often westward, though more pro- 

 nouncedly eastward because of a westward overthrust. 



THE EASTERN SECTION 



The Worcester district. — To the east of the central tract of little dis- 

 turbed schists, eastward overturning becomes pronounced in the Wor- 

 cester district, as exhibited in Perry's local section. The basins of Car- 

 boniferous and older rocks occupying the coastal lowland form parts of 

 an arcuate system of large folds concave toward a point near the heel 

 of Cape Cod, with a consequent turning of the line of the geological 

 cross-sections to the southeast. 



The Boston area. — In the Boston area the plan of the folding is com- 

 plicated by a fault along the northern border through Medford. The 

 sediments in the basin are accordingly turned up and locally overthrown, 

 as shown by W. M, Davis, at Maplewood, where the Roxbury conglom- 

 erate is cut out by an apparent upthrust, if not overthrust, of the crystal- 

 lines and the Lynn volcanics to the north of the fault. 



In the Cambridge argillites of the Mystic River quarries in Somerville, 

 overthrusting to the southeast is a pronounced feature in the detailed 

 structure of the argillite. Overthrusting in a selective manner on ap- 

 parently then unconsolidated clays also set in early, so as to produce a 

 zone of overturned and recumbent contortions about 30 feet thick in the 

 existing section. This movement was followed, after the intrusion of 

 ''^gray" dikes and sills, by thrust-planes on which the total movement 

 probably amounts to several hundred feet in the exposed section, single 

 thrusts measuring as much as 140 feet. 



Along the northern flank of the anticline of Roxbury conglomerate in 

 Brighton and Chestnut Hill, overthrusts with a southerly, large com- 

 ponent of motion have long been well known. Thus the prevailing over- 

 thrusting motion in the Boston basin is southward across the strike, 

 certainly not "away from the Atlantic basin.^' 



The Narragansett area. — The large structural feature of the eastern 

 arm of the Narragansett coal basin in Massachusetts is decidedly S3an- 

 metrical on either side of the central Taunton syncline, but in the north- 

 western corner of the basin, about North and South Attleboro, the 

 apparent structure about the Hoppin Hill granite block and its inclosing 

 Lower Cambrian formation is that of a flattened-down fan structure of 

 red Carboniferous beds overthrovm southward in a horseshoe-shaped fold 

 and overthrust upon the folded Coal Measures on the southern and south- 

 eastern border of this tract. In marked contrast ^ith this disturbance. 



