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 afeature in the fully developed ice of glaciers, and which has 
been expressively named by Professor James D. Forbes, the ribbon structure.* 
FOoLiATION. 
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. Suarpe, 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 lamine, formed by 
the crystalline constituents of the foliated rocks, are parallel to the planes or 
waves of heat 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 nevé is strictly parallel with the surface of the warmer mountain 
sides.” 
VOL. XXI. PART III. 6F 
