OF THE MORE DISTURBED ZONES OF THE EARTH'S CRUST. 451 



uniformly acting disintegrating forces, — either expansion and contraction by heat, 

 soakage and drying, or freezing and thawing, — could subdivide it by planes or 

 fissures, so regularly distributed as we find them. These could only have arisen, I 

 conceive, from the presence of parallel planes of weaker and stronger cohesion. 

 In this interesting structure, we discern a striking analogy to that alternation 

 of thin plates of solid blue crystal ice, and white porous ice of less cohesion, 

 which is so distinct a feature in the fully developed ice of glaciers, and which has 

 been expressively named by Professor James D. Forbes, the ribbon structure.* 



Foliation. 



The relations of the foliation or crystalline lamination of metamorphic strata 

 to the cleavage planes, and the planes of stratification, come next to be considered. 

 Two facts may be stated of foliation, which possess, perhaps, the constancy of 

 general laws. One of them is, that this structure, as it is seen in gneiss and mica 

 schist, observes, when the strata are not traversed by cleavage, an approximate 

 parallelism to the original bedding. Apparent exceptions to this rule occur in 

 several localities near Philadelphia, and elsewhere in the United States, and have 

 often been noticed in Europe, by Mr D. Sharpe, and other good observers ; but all 

 of them can be reconciled to the general fact, and reduced, it is conceived, to one 

 comprehensive law, namely, that the planes of foliation, or the laminse, formed by 

 the crystalline constituents of the foliated rocks, are parallel to the planes or 

 waves of heal which have been transmitted through the strata. Wherever large 

 tracts of the gneissic rocks retain a nearly horizontal undisturbed position, the 

 foliation is almost invariably coincident with the stratification, and in this case, 

 the wave of heat producing the crystalline structure can only have flowed upwards 

 through the crust, invading stratum after stratum, in parallel horizontal planes. 

 Again, when injections of granite occur in uplifted gneissic strata, the crystalline 

 lamination is generally seen to be parallel to the plane of outflowing temperature. 



* In a communication submitted to the American Association for the Advancement of Science in 

 1849, I attempted to show this analogy of the ribbon structure of glaciers to the slaty cleavage of 

 rocks, in the following remarks: — " The ice of glaciers consists of thin alternate parallel bands or 

 plates of blue crystal ice, and white porous ice, each not more than one-third or one-half of an inch in 

 thickness. These pervade the whole mass of every glacier, and are clearly exposed on the sides of 

 the transverse fissures. Near the sides of the glacier, they are almost absolutely parallel with its 

 mountain walls, but they sweep away towards its medial line, and form, like all the other planes which 

 divide the glacier, a series of innumerable loop-like curves. This looped or festooned form is obviously 

 caused in part by the downward tendency of the movement or flow of the semiplastic ice, and in part by 

 the influence of the terminal moraine to induce that parallelism to itself, which the rocky sides 

 of the glacier produce in the ice near them. The most general fact noticeable in relation to these 

 structural planes, is the approximate parallelism to the rocky walls and terminal moraine confining 

 the icy mass ; or in other words, to the surfaces of higher temperature, which inclose the glaciers. 

 However the direction of the ribbon lines may alter by irregularities in the onward flow of the glacier, 

 their position near the region of the neve is strictly parallel with the surface of the warmer mountain 

 sides." 



VOL. XXI. PART III. 6 F 



