Mr. HOPKINS, ON THK INTERNAL PRESSURE OF ROCK MASSES. 469 



evidence hitherto adcliiGed appears to be, on tlie whole, consistent with itself and strongly in favour 

 of the conclusion that whatever may have been the influence of pressure in producing a laniinatiil 

 structure, that influence must have been due to the tangential action parallel to those planes, and 

 not to direct pressure perpendicular to them. In fact, I regard the specimens above mentioned, 

 in which there is great angular distortion combined with an approximate coincidence of the planes 

 of cleavage and of stratification, as almost decisive against the latter conclusion. 



In the search of further evidence, the observer should direct his attention especially to those cases 

 in which the inclination of tlie cleavage planes to the bedding is either small or nearly 45°. In 

 the former case, according to the above inferences, he may e.xpect to find great angular distortion 

 of the fossils lying (as they will very generally be found to lie,) with the plane of separation 

 of the two valves parallel to the surfaces of the beds ; and in the latter case he may expect to 

 find the shells characterized more especially by direct compression or extension (more probably the 

 former,) in the plane of the bed, and in directions perpendicular to tiie intersections of that plane 

 with the planes of cleavage. At the same time it should be remarked that the angular distortion 

 may be accompanied by a lengthening or shortening of the .shell, (more probably the former.) in the 

 direction of the dip, and also that a considerable direct compression is not likely to be produced 

 without some degree of angular distortion; but still, if the above conclusions be true, angular dis- 

 tortion in the one case, and direct compression or extension in the other, ought e.specially to 

 characterize the actual forms of the organic remains. 



It might be objected against the theory to which the preceding conclusions tend to lead us, that 

 if tangential action has been an effective cause in the production of the laminated structure, there 

 ought to be two systems of cleavage planes at right angles to each other, since there are two such 

 systems of parallel planes in which the tangential action is a maximum ; and this might, I think, be 

 regarded as a valid objection to a theory which should assign the mechanical action here considered 

 as the primary cause of the laminated structure ; but the objection may probably be obviated in a 

 great degree, if we regard this kind of mechanical action only as a secondary cause, for it is very 

 conceivable that it might have greater effect in aiding the development of the structure in question 

 along one of the systems of planes of greatest tangential action than along the other. AVhatever 

 may be the apparent force of this objection, however, the discussion of it may be regarded, perhaps, 

 somewhat premature till further observation shall have ascertained more distinctly what indications 

 may be found of the existence of a second set of cleavage planes less developed than those which 

 more immediately attract our notice. The point is deserving of the attention of the geologist. 



The adoption of the opinion, that the mechanical agency above described has been one efficient 

 cause of the laminated structure, necessarily involves the conclusion of that structure having origi- 

 nated at some epoch posterior to the great movements which have determined the general configura- 

 tion of the external surface, and the geological structure of large portions of the eartii's crust, which 

 are observed to possess this laminated character. It is also a necessary inference tliat the line 

 of strike of the planes of lamination must coincide with that of the planes of stratification (Art. 7). 

 The results of observation undoubtedly corroborate this latter inference, for it would appear that 

 we may state as a general fact, that the strike of the planes of cleavage is jjarallcl to the directions 

 of the anticlinal lines of the district. The amount and character of the local deviations from this 

 law are not yet in any case, I believe, accurately determined. Observed facts appear, also, to 

 corroborate the above conclusion respecting the epoch at which the laminatcil structure was super- 

 induced ; for the persistency with which the strike and dip of the cleavage planes are frequently 

 maintained through disrupted and contorted strata, distinctly imy)lies that the lamination must have 

 lieen produced after the elevation and disruption of the general mass. These general facts are in 

 harmony with the theoretical view of the subject which has been here presented ; how far the more 

 detailed results of observation will be found so remains to be determined. 



Geologists are well aware that currents of electricity have been assigned as a probable cause of 

 the laminated structure, and that this hypothesis has received great support from the results of 



