B. D. Oldham — The Interior of the Earth. 21 



Especially in the case of the great thrust-faults is explanation 

 difficult ; the appearances are such as suggest a simple fracture and 

 displacement by compression due to approach to each other of the 

 limits of the region affected, hut it can easily he shown that the 

 thrusts involved in this explanation are many times greater than 

 those which could be transmitted by the material of which the 

 blocks are composed. The final explanation must wait until it can 

 be treated by one who is at the same time fully cognisant of the 

 geological results and of the physical principles involved, probably 

 also till a further advance is made in our knowledge of the physical 

 properties of the material under conditions such as exist, even at the 

 comparatively small depths involved. 



Leaving this question aside, it is clear that extensive displace- 

 ments have taken place in the outermost layers of the crust, and 

 these are presumably taken up, possibly in a somewhat different 

 form, by the lower layers, but in any case necessitate that, below 

 the rigid and solid crust, must come material which possesses some 

 of the properties attributed to a fluid, though not necessarily more 

 than the power of changing its form when exposed to stresses of 

 sufficient magnitude and of long enough duration. This has been 

 recognized for some time, and we were content to accept the general 

 conclusion without giving it a name, but this does not satisfy 

 American thought, and Professor J. Barrell has not only introduced 

 the name asthenosphere for the region of material comparatively 

 weak as against permanent stress, hut has given a numerical 

 estimate of this weakness or strength. According to him the 

 material at the weakest part of the asthenosphere reached by his 

 investigation, placed at 400 km. from the surface, is about - 2 -V of 

 that of massive surface rocks, and of the order of a capacity of 

 sustaining stress differences of about 1,0001b. per sq. in. with 

 extreme permissible limits of 100 and 5,000 lb. per sq. in. At this 

 depth the conclusions drawn from his method of deduction become 

 distinctly doubtful, but, at the lesser depth of 50 km., the strength 

 is only about six times that quoted and at 100 km. about four times 

 this amount. 1 



The nature of the transition from the solid crust, or lithosphere, 

 to the underlying asthenosphere is of interest, and on it two lines of 

 reasoning can be brought to bear. First, we have Professor Barrell's 

 calculations, according to which the permanent strength between the 

 depths of 20 and 30km. amounts to about four to five times that of 

 granite, and at a depth of 50 km. to only one-quarter, showing 

 a very rapid diminution of strength at depths below 30 km. and 

 a tolerably abrupt transition from the crust to the underlying 

 material. The other line of reasoning depends on the phenomenon 

 of reflection of earthquake waves. It is now pretty well established 

 that long distance records, when clear enough, show the arrival not 

 only of waves which have travelled from the origin by a direct path, 

 but of others which have been reflected at or near the surface. Jn 

 the mathematical treatment of these waves it is necessary to assume 



1 Journal of Geology, xxiii, p. 44, 1915. 



