ME. HOPEINS ON THE THEOET OE THE MOTION OE GLACIEES. 
729 
I may make another remark on this doubtful question. The dirt-bands of the Mer 
de Glace, if Dr. Tyndall’s views respecting them should be proved to be correct, may 
afford the means, supposing their forms and positions sufficiently determinate, of de- 
ciding how far the loops of structure in the middle and lower parts of the glacier may 
be merely the original transverse curves at its upper end, modified by transmission, or 
may be attributable to the great transverse pressure to which that glacier must be sub- 
jected at certain points of its course. If the dirt-bands are entirely formed at the upper 
part of the glacier and are there coincident with the curves of transverse structure, and 
if they also remain coincident with them at distant points of the glacier, then, since the 
dirt-bands must necessarily be transmitted forms, the curves must almost necessarily be 
so likewise. Principal Foebes’s theory of the dirt-bands would not lead to the same 
conclusion. 
59. The Pressure Theory and the Differential Motion Theory of the Veined Structure 
compared . — The pressure theory of the veined structure, so far as it asserts the perpen- 
dicularity of such surfaces to the directions of maximum pressure, appears to be in perfect 
accordance with observed facts and mechanical deductions, whether the structure be 
marginal, transverse, or longitudinal. The transversal directions and approximate verti- 
cality of the structural surfaces at the bottoms of ice-falls, and the general existence 
of the structure wherever the glacier must, from the conditions under which it is placed, 
be subject to great pressure, are also perfectly consistent with this theory. We may 
also remark that the evidence of facts in favour of it is in a great measure independent 
of the degree of efficiency which may ultimately be attributed to the transmission of the 
structural forms. I am not, indeed, aware of any leading observed facts of the veined 
structure inconsistent with this theory. 
With respect to the Differential Theory, the whole of the mechanical reasoning on 
which it is based is professedly popular, vague, and undemonstrative, and, I believe, as 
erroneous as such reasoning must almost necessarily be in cases as intricate as those 
which glaciers present to us, unless it be guided by an accurate conception and a careful 
analysis of problems which admit of more or less accurate solution, and are typical of 
those presented to us in nature. In the marginal structure, this theory could never agree 
with any very sensible deviation in the directions of the superficial curves of structure 
from parallelism with the sides of a canal-shaped glacier. The attempt to correct this 
defect by means of what has been called the E-ipple Theory, will not now, I imagine, be 
maintained by any glacialist. The constant existence of the veined structure under 
great pressure, and its comparative absence where such pressure cannot exist, are left by 
this theory without satisfactory explanation. It fails altogether to assign any direct 
cause for the great development of the longitudinal structure along the central moraine 
of compound glaciers like the Aar; for along the axes of such glaciers there cannot 
possibly be any differential motion which could produce it. The only structure which 
could there exist must be a transmitted structure. 
But the most conclusive objection to the differential theory is to be found, as I believe, 
