. A treatise on rocks, rock-weathering and soils;. Petrology; Soils. 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 ; 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
. A treatise on rocks, rock-weathering and soils;. Petrology; Soils. 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 ; 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 t
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectpetrolo, bookyear1913
