RHODE ISLAND OOAL MEASURES. 



167 



45° E. In the railway cut the same series of strata show a north-south 

 vertical fault plane, the slickensides of which are horizontal. There is 



Fig 21 —Geological section through rocky islets at Halsey Farm, Silver bprmg a, the rocky head shown in PI VI; 



R R , the railroad 



present a coarse slaty- cleavage, but it is not a constant feature. The 

 joint planes display great feathery surfaces of fracture, the divergent 

 lines of which indicate the direction of splitting; this sometimes is upward 

 or downward, but very frequently in a horizontal direction, the plane being 

 vertical With certain precautions, the dip of massive beds can be obtained 

 by observing the inclination of the axes of these feather fractures. The 

 joint planes frequently die out with a convex front, the periphery being cast 

 into concentric flexures of conchoidal fracture, often measuring an inch from 

 trough to crest. These planes of fracture exhibit splitting figures having a 

 length of at least 6 feet from the rather indistinct point of origin to the 

 sharply incised marginal nigosities.^ 



A few rods farther south, but north of Pomham Rock, is a small rocky 

 headland, composed of two distinct ridges, in which the following section 

 was shown, dipping 45° E., the same rocks reappearing in the islets north- 

 ward by the Silver Spring shore. 



6 <y 



Fig 22.— Geological section of rocky headland below Halsey JFarm. Bluff, near Silver fepnng, Rhode Island, (For expla- 

 nation of letters, see accompanying section.) 



Seetion in rooky headland near Silver Spring Station^ Rhode Island. 



reet. 



e. Slate (at top) 15 



d. Grits 15 



c. Slates, partly covered 25 



b. Grits and sandstones, with pebbles, isolated, and in bands and pockets 15 



a. Slates witb sandy layers, showing local unconformity to b, due to contempo- 

 raneous erosion; plant stems occur, and the upper portion of the bed is car- 

 bonaceous; cross bedding very marked; exposed above high-tide mark 6 



iFor a detailed description of this type of fractures, see paper ^' On the ixactnre system of joints, 

 3to.," by the author: Proo. Boston Soo. Nat. Hist , Vol. XXVII, 1896, pp. 163-183 



