ON AN EXPERIMENTAL INVESTIGATION INTO THE FLOW OF MARBLE. 387 
throughout its mass. In general these slips no doubt occur in three planes or 
possibly more, and the combination of the three allows the grain to accommodate 
itself to its envelope of neighbouring grains as the strain proceeds. The action is 
discontinuous; it is not a homogeneous shear but a series of finite slips, the portion 
of the crystal between one slip and the next behaving like a rigid solid. The process 
of slipping is one which takes time, and in this respect the aggregate effect is not 
easily distinguishable from the deformation of a viscous liquid. 
“We infer from the experiments that ‘flow’ or non-elastic deformation in metals 
occurs through slip within each crystalline grain of portions of the crystal on one 
another along surfaces of cleavage or gliding surfaces. There is no need to suppose 
the portions which slip to be other than perfectly elastic. The slip when it occurs 
involves the expenditure of work in an irreversible manner. It is because the metal 
is an aggregate of irregular crystals that it is plastic as a whole, and is able to be 
deformed in any manner as a result of the slips occurring in individual crystals. 
Plasticity requires that each portion should be able to change its shape and its 
position. Each crystalline grain changes its shape through slips occurring within 
itself, and its position through slips occurring in other grains.” 
By a comparison of these results in the deformation of metals with those presented 
in the present paper in the deformation of marble, it will be seen that the agreement 
between the two is so close that the term “flow” is just as correctly applicable to the 
movement of heated marble in compression, under the conditions described, as it is of 
the movement which takes place in gold when a button of that metal is squeezed flat 
in a vice, or in iron when a billet is passed between a pair of rolls. 
V. Comparison of the Structures produced in Carrara Marble by Arti¬ 
ficial Deformation with those observed in the Limestones and 
Marbles of highly contorted portions of the Earth’s Crust. 
While the microscopic structure of the silicated rocks of the earth’s crust has been 
made the subject of most exhaustive researches during the past half-century, com¬ 
paratively little attention has been paid to the minute structure of limestones and 
marbles. The papers which have appeared on this subject deal chiefly with unaltered 
limestones, and there is but little mention made of structures resulting from pressure. 
Pfaff,* in his somewhat extended study of limestones and dolomites from many 
widely separated localities, in which lie examined some 700 thin sections of these 
rocks, representing, however, chiefly unaltered strata of Mesozoic or Neozoic age, does 
not mention a single instance in which cataclastic structure was observed, and states 
that in only two instances was a flattening of the calcite grains seen, producing a 
species of foliation in the rock. Both of these were from the Alps, one of them a 
* “ Einiges iiber Kalksteine und Dolomite,” ‘ Sitzber. tier Math.-phys. Classe tier K. Bayer. Akad. 
der Wissen. zu Munchen,’ 1882. Heft 4, p. 566. 
3 D 2 
