i66 Mb. HOPKINS, ON THE INTERNAL PRESSURE OF ROCK MASSES. 



and therefore the greatest tangential force 



= ^{A + C)=f, 

 and it acts in the planes FQ and FS ; i. e. it is the impressed tangential force, as it is sufficiently 

 manifest it ought to be. 



(6) It is of some importance with reference to the particular application of these investigations 

 which is here contemplated, to remark that when the general mass is so acted on by external forces 

 that its different elementary portions are subjected in very different degrees to the kind of distortion 

 represented in fig. 3., there may be a great extension or compression at particular points without 

 a correspondent increase or decrease on the same scale in the general dimensions of the mass. 

 Indications of such local extension and compression seem to be frequently indicated by the distortion 

 of organic remains. 



SECTION II. 



7. Organic remains, more especially shells, are usually found in greatest abundance along 

 those surfaces within a fossiliferous mass, which we recognize as planes or surfaces of separation 

 between contiguous beds. These shells, especially the flatter ones, will generally be found with 

 their flatter surfaces parallel to the surface of the bed on which they lie, and such may also be 

 expected to be the case as a general rule, with respect to shells contained within a bed instead of 

 beinn- between two contiguous beds. The first pressure to which these shells was subjected must 

 have been that due to the weight of the superincumbent beds deposited upon them, while the whole 

 remained undisturbed. If the shell yielded to this pressure it would become flattened, and fre- 

 quently also extended in length or breadth, or in all directions according to the nature of the shell. 

 It would seem probable that the proportions of the linear horizontal dimensions would not be 

 mucli altered by this vertical compression, but the possibility of its being otherwise should not be 

 forgotten by the observer. It may also be remarked, that should any horizontal elongation take 

 place from this cause in one direction more than another, that direction can only have reference to 

 the shell itself, and not to any fixed lines in space, unless it can be shown that the position in which 

 the shell was originally imbedded bore some relation to such lines, as for instance, that the median 

 lines from the beak to the margin in different shells should have been parallel to some common direction. 

 Any such law, however, would seem to carry with it the highest degree of a priori improbability. 



When the mass became elevated and dislocated, especially in the degree in which such has been 

 the case in most of the ancient fossiliferous rocks, it would generally be subjected to great 

 pressures and tensions; but it is of the first importance to remark, that none but comparatively 

 small pressures or tensions could be called into action in the direction of the strike of the beds, 

 by their elevation into straight, or approximately straight anticlinal ridges; and that, consequently, 

 two of the directions of principal tension or pressure must lie in a vertical plane perpendicular to 

 the direction of the anticlinal line and strike of the beds, with which the third axis of principal 

 tension must coincide. Now in this elevation, it is highly probable that the mass will generally 

 be extended in some directions, and I consider it almost certain that it must, in most cases, be 

 compressed in other directions, these compressions and extensions taking place in the above 

 mentioned vertical plane perpendicular to the strike of the beds. Hence, we may conclude that 

 generally the mininnim tension will be a pressure, as in (.3) of last Article. The axes of greatest 

 and least tension through any point will lie in a vertical plane perpendicular to the strike of the 

 beds, and consequently the intersections of the planes of greatest tangential action with the planes 

 of the beds will be horizontal lines. Through every point there will be two planes of maximum 

 tanirential action perpendicular to each other, and therefore, dipping one of them in the same 

 direction as the beds, and the other in exactly the opposite direction, the strike of all these planes 

 being the same. 



