166 RALPH S. TARR 



ment of abundant joints in the dike and a slight disintegration along 

 them. A similar case occurs where a large porphyritic diabase dike 

 crosses a quarry at Pigeon Cove ; but here residual clay remains along 

 the joints, by which the dike is separated into a series of rounded 

 bowlders. 



"Sap." — In all the quarries at Cape Ann the granite is stained 

 yellow along both the vertical and horizontal joint planes. This 

 stain, which in some cases extends three inches into the granite, is 

 known by the quarrymen as "sap." It is due to a partial disintegra- 

 tion of the iron-bearing minerals and consequent staining of the 

 granite. The depth to which this stain extends in the quarries, being 

 found even at the very bottom of the largest, and the extent to which 

 it enters the rock, are altogether too great for postglacial decay. 



Joint planes. — Almost uniformly over the Cape it is the case that 

 joint planes are much more numerous in the upper than in the lower 

 portions of the quarries. 1 They are in some places so close together 

 that small quarries have been opened and, after being worked for 

 a while, have been abandoned because the "grout" (small irregular 

 blocks stained with "sap" and useless even for making paving 

 blocks), which must be handled and disposed of, is too abundant. 

 These joint planes have evidently been developed by weathering, 

 and by weathering of longer duration than postglacial times would 

 permit. Glacial erosion has not been sufficient to remove this upper 

 zone of jointed granite, although the joints produce masses favorable 

 for plucking. 



Inter glacial ( ?) beds. 2 — At Stage Fort, just outside of Gloucester, 

 there is a bed of fossil-bearing sands, clays, and gravels, overlain 

 by till and grooved by glacial erosion. The layers are crumpled 

 by the ice-shove. Although in the lee of a range of hills, it would 

 hardly be expected that such a deposit would escape vigorous erosion. 



Decayed granite. — In Lanesville, on the stoss slope of the Cape, 

 and in a situation where there is no topographic reason for protec- 

 tion, there occurs in the Edwin Canney quarry a striking instance 

 of decay in the granite (Figs. 4 and 5). 3 This decay, which extends 



J See Plates LXVI and LXVII in Ninth Annual Report, U. S. Geological Survey, 

 1887-88. 



2 Tarr, Bulletin of the Museum of Comparative Zoology, Vol. XLII (1903), p. 189. 



3 See also Plate LI, Ninth Annual Report, U. S. Geological Survey, 1887-88. 



