All Unrecognised Agent in the Deformation of Bocks. 397 



must be positions of pressure along which they will yield most 

 easily. If then any granular rock composed of crystalline substances 

 is subjected to slight stresses, at the same time is allowed to remain 

 in this position for a sufficient period, the pressure will find out these 

 directions of easy yield, and the rock will be thereby distorted. 



Another view of the question is whether under sufficient pressure 

 a rock will accommodate itself to stresses in the same way as when 

 time is one of the elements in the process. Prof. Daubree has 

 recorded several experiments of this nature. For instance, he rolled 

 a coin between the rollers of a machine for making sheet-iron, and 

 produced a lengthening like that one often sees in fossils which have 

 been subjected to pressure ; he also placed a piece of chalk between 

 two layers of lead and subjected them to immense pressure, the lead 

 flowed, broke the chalk into bits and separated them." The object 

 of my experiments was to try and get such a high pressure that the 

 solid bits subjected would be unable to fracture, at the same time to 

 give the mass a differential movement sufficient to produce distortion. 

 For the purpose some limestone pebbles, and a pencil of hard shale 

 were imbedded in lead, the whole encased in a strong iron box. 

 Through the courtesy of Mr. G. McGrath of the Salt Eiver Works, 

 the box was then put in a hydraulic ram, and subjected to a pressure 

 of 70 tons, when the box began to give ; eventually 75 tons was 

 reached, when the box almost burst. The diameter of the box was 

 ^:5"55 inches, giving an area of 11"15 square inches, so that the 

 pressure per square inch was respectively 6*28 and 6*72 tons. Since 

 the pressure in the interior of the box was the same in all directions, 

 the rocks imbedded in the lead were subjected to this pressure. On 

 opening the box and cutting out the stones it was found that the 

 pressure was below that necessary to keep the parts of the rock 

 in continuity and the experiment failed in its principal object. I hope 

 at some future date to try the same experiment with a steel box 

 sufficiently strong to get a pressure of 20 to 25 tons per square 

 inch. It will be noticed (fig. 2, pi. iv.) that the pencil of slate is 

 most distorted not in the zone of the greatest bulging of the 

 lead, but considerably above this. The explanation of this is to be 

 found in the way in which the box was filled. The lead was first 

 poured in till the box was half full, and the specimens were placed in 

 position, then another layer was added, and some pebbles placed in, 

 and finally the whole was filled up. The first two pourings developed 

 a small amount of dross on their surface as they cooled, and under 

 pressure the layer of lead between these two surfaces remained 

 separated from the rest, the consequence was that while the rest of 



* " Geologie Experimentale," pp. 420-2. 



