Shaler.] 290 [February 3, 



their way beneath the successive concentric layers of the rock, rend 

 them In large masses from their places and grind them to pieces. 

 The fragments are then either carried to deeper water, where they are 

 protected from further effects of the waves, or impelled by the action 

 of storms striking the shore obliquely, they are drawn to the nearest 

 nook of tlie coast, where they are dragged to and fro among the rat- 

 tling pebbles of a rolling beach until reduced to sand or mud, and borne 

 away by tidal currents. Thus, owing to this structure, massive rocks 

 are rapidly worn away which would otherwise present an almost un- 

 yielding front to the waves, and the deposit of sedimentary matter 

 proceeds with rapidity over areas Avhich would receive little such in- 

 crement were it not for this element of weakness of the neighboring 

 rock shores. It is unquestionable that the rapid silting up of the 

 numerous inlets and bays which fret the shore of the northern and 

 southern hemisphere within the so-called fiord zone, a phenomenon 

 of political, as well as scientific Importance, Is due in a considerable 

 degree to the rapid decay of the massive rocks, rendered possible 

 by this feature in their structure. 



Concentric lamellation differs widely from the common features of 

 cleavage in rocks, inasmuch as however complicated and distorted the 

 cleavage system may be, it is always reducible to sets of planes, 

 crossing each other, — if there be more than one such system, but 

 never producing systems of curves, which are the essential feature in 

 the fractures we are describing. This much is readily seen upon the 

 exterior of any mass characterized by this structure. Uj^on examin- 

 ing, where it has proved possible, the internal features, the interesting 

 fact became evident that the concentric arrangment was confined 

 to the external portions of the mass, never being discernible at a 

 greater depth than four or five feet, rarely. Indeed, below -three feet 

 froi;i the surface. This determination has been made from the exam- 

 ination of a very few sections, which were fitted for the purjjose, 

 Inasmuch as It is by no means easy to find quarries which give suffi- 

 ciently extensive sections to admit of study of such features, which 

 cannot be well examined in a small sectional area. Sometimes it 

 happens that at greater depths than are. above indicated, there occur 

 fissures which at first sight seem to have the same general corre- 

 spondence with the surface, as the concentric or onion fracture in 

 question; but it has always happened that a careful examination has 

 rendered it appai-ent that the correspondence Avas accidental, and the 

 lines not of the concentric character at all. It may be possible in 

 case small dome-like elevations exist In natui'e, to have phenomena 



