﻿Vol. 64.] QITANTITATIVE METHODS TO THE STUDY OF EOCKS. 227 



In some places east of Sheffield similar surfaces of interlocking 

 teeth, separated by a thin layer of earthy residue, are met with 

 in the Magnesian Limestone, which well deserve further study, 

 and might show that it is necessary in the case of dolomites to 

 take into account other circumstances besides pressure and simple 

 solution. 



XIX. Determination oe the Peessuee to which Rocks 

 HAVE BEEN Subjected, 



That the pressure brought to bear when mountains were elevated 

 and slaty cleavage developed was very great will readily be admitted, 

 since it causes fairlj^-hard rocks to yield as if more or less plastic. 

 It appeared to me, however, unsatisfactory to remain content with 

 merely calling it a great pressure, and not to attempt to form some 

 estimate of its value in tons to the square inch, or the weight of so 

 many feet of superincumbent rocks. I therefore attacked this 

 problem, both by experiment and by the discussion of my observations 

 made with different rocks. 



My first experiments were made in strong brass tubes, '6 inch in 

 internal diameter, into which a solid brass rod fitted. Finely- 

 powdered dry pipe-clay was used, since the presence of water would 

 have greatly increased the difficulties. The tube was filled with 

 this clay and it was compressed by hand with the brass rod, and so 

 much added as to make the length of the column of clay 2 inches. 

 Then gradually increasing pressure was applied, and the reduction 

 of volume determined by measurement. The weight of water filling 

 the tube up to 2 inches and that of the clay were known, and also the 

 specific gravity of the solid material of the clay, from which it was 

 easy to calculate the percentage of empty spaces, when variously 

 compressed. Taking the solid volume of the clay as 100, that of 

 the interspaces was, when pressed by hand, 76*7 ; when the brass 

 rod was driven in by moderate blows of a 1 lb. hammer, it was 

 34 per cent. ; and with hard blows, 21*4 per cent. This method, 

 however, did not enable me to estimate properly the pressure. 

 This was attempted by forcing in the rod by a screw, with a 

 pressure estimated at not much short of 10 tons, or about 30 tons 

 to the square inch, which is equal to the pressure of about 75,000 

 feet of superincumbent rock.- In this case the amount of inter- 

 spaces was 29 '2 per cent. It is, however, probable that it would 

 have been a good deal less, if the pressure had been continued for 

 a long time. These experiments, therefore, merely show that, in 

 some way or other, the interspaces vary inversely as the pressure. 



The Table (YIII) on the following page is a list in descending 

 order of all my determinations of the interspaces in clays, shales, 

 and slates, adopting what seem to be the most probable results. 

 The amounts are percentages of volume. 



