714 
IMR. HOPKINS ON THE THEOEY OF THE MOTION OF GLACIEES. 
the mass is crystalline and brittle, that we see best, perhaps, the distinction between the 
Viscous and Regelation Theories. 
Section V. — On the Veined Structure of Glacial Ice, 
40. Two ditferent theories, as is well known, have been proposed to account for the 
curious structure frequently observed in glacial ice, and termed the laminar, veined, or 
ribbon structure. The preceding investigations bear intimately on these theories. One 
of them, that of Principal Foebes, asserts the structure to be due to the sliding of one 
thm lamina of ice past another, or to their differential motion — a process by which he 
supposes extensive portions of the mass to be divided into slices, usually nearly vertical, 
and not exceeding, in many cases, the fraction of an inch in thickness. His first idea 
was that these parallel discontinuities were, after their formation, filled with infiltrated 
water, which, by being frozen, formed the veins of blue ice. He afterwards appears to 
have abandoned this notion ; but what physical process he supposed to be substituted 
for infiltration I have not been able distinctly to ascertain. This, however, is not 
material as regards the examination which I propose to give this theory in the sequel. 
The other theory above alluded to is that first proposed by Dr. Tyndall, an essential 
point in which is that the laminge of ice which characterize this structure must be per- 
pendicular at each point to the direction of greatest pressure there. So far alone my 
researches are related to this theory, and so far only shall we be here concerned \vith it. 
I shall not discuss the manner in which this pressure is supposed to produce the lami- 
nation in question, a point on which somewhat different opinions have been pro- 
pounded. It is the mechanical part only of the theory that I shall discuss. It has 
been called the pressure theory of the veined structure, while that advocated by Prin- 
cipal Foebes has been termed the differential theory of the structure. The lines in 
which the laminae of blue or white ice meet the surface of the glacier will generally be 
cuiTed lines, which I shall designate as lines or curves of structure. 
This structure, as well as the lines depending upon it, have received different designa- 
tions according to the portion of the glacier where it is found, or the directions of the 
lines of structure. Thus, when found in the lateral portions of the glacier. Dr. Tyndall 
has termed it the marginal structure. It is frequently found in canal-shaped glaciers. 
The lines of structure in such cases are usually inclined at small angles to the sides of 
the glacial valley. When the lines stretch more directly and entirely across the valley, 
the structure is said to be transverse., and is more especially developed at points below 
an ice-fall aud not remote from it. In the axial part of a glacier the lines of structure 
are frequently longitudinal, or parallel to the axis. The structure is then called longi- 
tudinal. It is generally best exhibited at the confluence of two large glaciers, as those 
of the Finsteraar and Lauteraar. It may be convenient to preserve these designations, 
though, as will be seen, they are the results, according to the pressure theory, of the 
same general cause, modified by the local conditions under which it acts. I take again 
