230 THE PHYSICAL MANIFESTATIONS OE WEATHEEING 



the finer grained, hard, and nndecayed 'greenstone' constituting 

 the outer portions of the dike rises up in the shape of ridges 

 and chains of islands, so that the river flows as a main, central 

 channel, more or less separated from the smaller lateral ones." 

 The same writer describes several instances in which long straight 

 valleys in the Archaean regions of Canada, now occupied by 

 straight river stretches, long narrow lakes or inlets of the larger 

 lakes, are due to the decay and removal of the wide ** greenstone" 

 dikes, or of parallel dikes with narrow belts of rock between. 

 Long Lake, north of Lake Superior, some 52 miles in length, is 

 mentioned as typical of lakes of this class. 



(3) Weathering influenced by Structure of Rock Masses. — 

 In any rock mass weathering is greatly augmented by lines of 

 weakness, such as joint and bedding planes, since these furnish 

 so many additional points of attack. In homogeneous massive 

 rocks the rate of disintegration is retarded by a lack of vulner- 

 able points, and the resultant form is that of rounded bosses 

 such as are shown in plate 1. 



As a rule, however, the most massive of rocks are traversed 

 by one or more series of joints (see Pis. 16 and 19) whereby they 



EiG. 17. — Showing the influence of joints in tlie production of boulders. 



are divided up into rhomboidal blocks of varying sizes. Even 

 when not sufficiently developed to be conspicuous, such joints 

 may exist as lines of weakness along which moisture and the ac- 

 companying agents of disintegration make their way, gradually 

 rounding the corners until there are left but the oval boulder- 

 form masses of which the so-called ''niggerheads" of the gabbro 

 area about Baltimore are typical examples. In nearly all such 

 rocks the exfoliation and decomposition take place in the form 

 of concentric layers, like the coatings on an onion. This holds 

 true with the huge granitic bosses, as well as with the smaller 



