COHESIVE ATTRACTION. 629 



would produce rings among flattened concretions, or in clayey layers. 

 The solidification commences first at the circumference, and then the 

 circle thus begun acts as a nucleus about which concreting is continued. 



The concentric coats in many concretions are due to an intermittent 

 action in the concreting process. If a drop of a weak solution of sugar 

 dry upon a slab of stone in the air, concentric rings result. The outer 

 edge of the circular spot dries most rapidly ; and, when solidification 

 begins along it, the liquid inside for a limited distance is drawn to the 

 concreting circle, exhausting the sugar for that distance inward ; then 

 the spot of dissolved sugar, thus made smaller, concretes again at its 

 outer edge, and forms in the end a new circle ; and so it goes on until 

 all is evaporated. A concentric arrangement of colors and of layers 

 is often thus produced in ferruginous concretions, the outer shell first 

 drying and concreting, and afterward successive concentric shells, to the 

 centre. 



(4.) Resistance to Fracture. — Rocks owe their power to resist frac- 

 ture under pressure, first, to the nature of the grains: their hardness, 

 cleavability, shape and arrangement, size if cleavable ; and secondly, 

 to the nature of the mass : the strength of cohesion in it, closeness of 

 texture, degree of schistosity, and degree of elasticity. The trials of 

 General Gilmore with American rocks showed that trap (doleryte), 

 although not the hardest, is one of the firmest of rocks. He found the 

 weight required, per square inch, to crush New Jersey trap, 20,750 to 

 24,040 lbs. ; for granite of Westerly, R. I., 17,750 lbs. ; of Richmond, 

 Va., 21,250 ; of Fox Island, Me., 15,062 ; for syenyte of Quincy, Mass., 

 17,750; for sandstone of Portland, Ct., 6,950; of Belleville, N. J., 

 10,250; of Berea, O., 8,300; of Amherst, O., 6,850; of Dorchester, 

 N. B. (the olive green rock), 9,150; of Medina, N. Y., 17,250; for 

 limestone (black) of Glens Falls, N. Y., 11,475 ; of Joliet, 111., 1 1,250 ; 

 of Caen, France, 3,650 ; for marble of Tuckahoe, N. Y., 12,950 ; of 

 Dorset, Vt., 7,612 ; statuary, of Carrara, Italy, 9.723. Gen. Frank- 

 lin found the weight required to crush the marble of Pittsford, Vt., 

 with two specimens, 9,028 and 12,239 lbs. 



P. Michelot obtained for the crushing-weight of basalt of Estelle, 

 France, 26,830 lbs. ; of good granite of France, 14,330 to 21,500; of 

 the best compact limestones, 12,900 lbs. ; of the best marbles, 7,160 to 

 10.000 lbs. ; of trachytes, 5,170 to 13,600. 



Wet, absorbent rocks, crush much more easily than dry, wet chalk 

 requiring but one third that of dry. This easy slipping of the grains 

 is a mechanical effect in part, but is often owing also to the water dis- 

 solving a slight portion of the material. 



2. Capillary Attraction. — The process resulting in concentric 

 coats, described in a paragraph above, is due partly, as is seen, to cap- 



