322 CALIFORNIA ACADEMY OF SCIENCES. 



In Other words, vertical pressure is not necessary to 

 prevent the breaking of folding beds of rock when time 

 enough is given for a small horizontal force to act. It 

 does not concern us now whether this be true if carried 

 to its smallest limit. 



No experiments on rocks covering an}- extended period 

 of time seem to have been carried on," but a few cases 

 have been observed in which most of the conditions of 

 such an experiment have been fulfilled and sufficient time 

 has elapsed to make the results pronounced. ^^ In but 

 one of these have measurements been made and pub- 

 lished. ^^ 



The following notes on a slab of marble bending under 

 its own weight are offered as a contribution to observa- 

 tions upon this subject. The slab in question covers a 

 grave in the Laurel Hill Cemetery, San Francisco, not 

 far from the corner of California Street and Central 

 Avenue. It is three-fourths of an inch thick, six feet 

 four inches long, two feet six inches wide and lies in a 

 horizontal position. As far as could be determined, the 

 slab was laid upon the prepared earth, which at this point 

 is clay, without support of brick, cement or other ma- 

 terial. 



It could not be learned just when the slab was put in 

 place, but it was not earlier than 1882, nor later than 

 1884. Subsequently the ground settled, and the slab 

 being then supported only near the ends, bent as shown 

 in the illustrations. 



The position of the supporting earth could not be de- 

 termined, but it probably occupies an irregular area near 



11 Miall in Pop. Sci. Keview, Jan. 1872. 



'^ Becker in Bull. Geol. Soc. Am., Vol. 4, 1893, p. 51; King, U. S. Geol. 

 Exp. 40tli Par., Vol. 1, p. 752. 



'•' Winslow, Am. Jonr. Sci. Ill, Vol. 43, 1892, p. 133. 



