468 Mr. HOPKINS, ON THE INTERNAL PRESSURE OF ROCK MASSES. 



9. The application of these conclusions to the leading object of this communication, the possible 

 influence of internal pressure in producing cleavage structure, may be made very briefly. If we 

 recotrnize the probability of this influence, if not as a primary cause, yet as effective in determining 

 the positions of the planes of cleavage, we must, I think, almost necessarily suppose, as I have 

 before remarked, that those planes must coincide more or less accurately either with planes 

 perpendicular to the directions of maximum pressure, or with those perpendicular to the direc- 

 tions' of minimum pressure, or with the planes of greatest tangential action. Now, let us sup- 

 pose the organic forms lying on the surface of a bed to have suffered great angular distortion, 

 and therefore the planes of stratification and of greatest tangential action to have been at least 

 approximately coincident ; then, if the planes of cleavage nearly coincide with those of stratification, 

 we may conclude that the tangential action and not the direct pressure or tension has been the effective 

 agency in determining the position of the cleavage planes ; and the conclusion will be strengthened 

 if we find that, as a general rule, the angular distortion is greater the more nearly the planes of 

 stratification and of cleavage are coincident. Again, suppose the observed distortions to consist in 

 direct compression or extension, without considerable angular distortion, and therefore the planes of 

 stratification to have been perpendicular either to the directions of greatest pressure or to those of 

 greatest ten.sion, and consequently inclined at an angle of 45° to the planes of greatest tangential 

 action ; then, if the cleavage planes be also inclined at an angle of nearly 45° to the planes of stratifi- 

 cation, we shall be again led to the same conclusion as above. If, on the contrary, it shoulil be found 

 that wlien the cleavage planes and the planes of stratification are nearly coincident, the distortion con- 

 sists only in direct compression ; or if, with great angular distortion, the cleavage planes should be 

 inclined at about 45° to those of stratification (cases exactly opposite to those previously supposed,) 

 we must conclude that direct pressure has been the influential cause in determining the position of 

 the planes of cleavage. 



In the memoir already referred to, Mr. Siiarpe has collected, I believe, nearly all the evidence 

 which has hitherto been obtained on this subject, consisting principally of observations made by 

 himself and Professor Phillips, and has given drawings of several characteristic distortions, principally 

 of spirifer clisjiinctus, a frequent and well-known shell in some of the older formations in which the 

 cleavage structure is very distinctly developed. In the most remarkable specimens of Mr. 

 Sharpe's collection (for the inspection of which I am indebted to him) the distortions are very striking^, 

 and, for the most part, of that kind which I have termed angular 'distortion. Now all the most 

 remarkable instances of this kind, as Mr. Sharpe has stated in his memoir, are those in which the 

 planes of stratification and those of cleavage are approximately coincident, the angles between 

 them varying from one or two to ten or fifteen degrees; whence 1 should conclude that the 

 cleavage planes must have approximately coincideil with ihe planes of greatest tangential actixm, 

 and consequently that it is to this kind of mechanical action, and not to direct pressure, that 

 the influence in the production of the cleavage structure must be attributed. Mr. Siiarpe has 

 al.so described and figured other specimens taken from beds in which the planes of stratification 

 are inclined to those of cleavage at angles varying from forty to sixty degrees, and in these cases 

 the distortions (as described in his memoir) consist in a shortening of the axes of the shells in 

 directions perpendicular to the intersections of the planes of stratification with those of cleavage, 

 such as would result from direct pressure in that direction. So far this evidence is perfectly in 

 accordance with that ))reviously cited, for it indicates that the direction of maximum pressure must 

 have approximately coincided with the planes of stratification, and therefore that these planes must 

 have been inclined approximately at an angle of forty-five degrees to those of maximum tangential 

 action. Consequently these latter planes must have approximately coincided with the cleavage 

 planes in this case as well as in the former one. This latter evidence, however, furnished by Mr. 

 Sharpe's specimens is not, probably, nearly so complete with respect either to the number of dis- 

 torted shells or the distinctness of their distortions, as that furnished by the shells first men- 

 tioned as so curiously and distinctly characterized by great angular distortion. Still, the 



