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MANSFIELD: ROXBURY CONGLOMERATE. 219 
Farther south they are found in the vicinity of the Arnold Arboretum, 
at Jamaica Plain, and farther east at Squantum. The zones thus 
roughly indicated have already been shown to represent areas of 
folding or dislocation and the occurrence of these distorted pebbles 
in such regions shows that their production was closely related to 
these movements. 
One of the most conspicuous features of the conglomerate is the 
remarkable development of joints. The latter usually occur in two 
main systems approximately at right angles to each other and hav- 
ing approximately north-south and east-west directions respectively, 
though the latter sometimes vary as much as twenty or thirty degrees 
on either side of these positions. Their dips are almost invariably 
Steep, often vertical. Sometimes they are accompanied by joints of 
intermediate strike and dip. The noteworthy feature of these joints, 
aside from their constant occurrence and direction, is the:smoothness 
and regularity of the surfaces produced by them. The rock is sliced 
as though with a knife, so that pebbles and matrix, hard and soft 
materials alike, are cut through with equal ease and precision. Simi- 
lar joints in the conglomerate at Newport were long ago noted and 
described by C. H. Hitchcock (a, p. 113-114), who attributed their 
smoothness to the plastic condition of the rocks when they were forined. 
It is certain that their presence indicates conditions of great pressure 
or strain so that it is probable that their production was an accom- 
paniment of the deformation process by which the rocks of the basin 
attained their present structure. 
Tur NorrouLk Basin.—Literature. The Norfolk Basin sedi- 
ments have received much less attention in geological literature 
than those of either the Boston or the Narragansett Basin, largely 
on account of the extensive drift covering and the somewhat un- 
Satisfactory nature of the exposures. The earlier papers of Presi- 
dent Hitchcock and W. W. Dodge paid some attention to this 
area; the former, from the red color of the rocks, suggested their 
€quivalence to the Old Red Sandstone. Crosby, in his Contribu- 
tions to the Geology of Eastern Massachusetts, touched upon the 
Norfolk Basin sediments and expressed the opinion that they are 
Synchronous with the rocks of the Boston Basin, which he then 
believed were Primordial. Later work by Crosby and Barton 
(1880) established the connection of the Norfolk Basin sediments 
with those of the Narragansett Basin and called attention to the exist- 
ence of certain casts of tree trunks of Carboniferous age in the vicinity 
