Mk. HOPKINS. ON THE INTERNAL PRESSURE OF ROCK MASSES. 



•16 



8. Let us now consider how the distorted forms of organic remains may indicate the 

 directions which must have been 

 those of maximum and niininium C^'S- *■) 



tension or pressure, and the po- 

 sition of the planes of maximum 

 tangential action at some former 

 epoch, posterior to the elevation 

 which raised the general mass 

 into anticlinal ridges. In the 

 first place, suppose the planes of 

 maximum tangential action to 

 coincide, at least approximately, 

 with those of stratification. Let 

 MN represent one of these planes 

 on which, between two beds, the 

 fossil shell ^B is found, the un- 

 distorted form of the shell being 

 known. MN is supposed to co- 

 incide with the dip of the beds, 

 and the median line of the shell to lie in the direction of their strike, the plane of the paper 

 being vertical. Also, let CT and CP be the directions of maximum and mimimura tension 



<Fig. 5.) 



respectively, each inclined at an angle of 45" to MN. 

 Then the continuous line will represent the distorted 

 form of the shell, of which the original form is indicated 

 by the dotted line. Fig. 5, in which the plane of the 

 paper represents the plane of a bed, will represent the 

 distorted form of the upper valve of the same shell. It 

 is important to remark, that this angular distortion will 

 take place in the direction of the dip {MN) of the beds, 

 and perpendicular to their strike {SCS). 



Again, let the planes of stratification be perpendicular 

 to one of the directions of principal tension, then will 

 MN the direction of the dip, be a direction of maximum 

 tension or of maximum pressure In the former case an 

 imbedded fossil will be elongated, and in the latter case 

 compressed, in the direction MN, but without any of 

 that angular distortion represented in the previous case 

 (Fig. 4), unless it should be accidentally produced by 

 direct compression, in which case, however, it will have 

 no such necessary reference to the directions of dip and 

 Btrike as above mentioned. 



Conversely, if it be observed that the organic forms 

 lying between two contiguous beds, have undergone 

 great angular distortion, we may conclude that the 

 planes of stratification must have coincided more or less, 

 approximately, with those of maximum tangential action 

 produced; but if the observed distortions indicate only direct compression or extension, unac- 

 companied liy angular distortion, we may conclude, that the planes of stratification, at the lime 

 juRt mentioned, must have coincided at least approximately, with the directions of maximum or 

 of minimum pressure. 



at the time when the distortions were 



