This section is from the book "An Introduction To Geology", by William B. Scott. Also available from Amazon: An Introduction to Geology.
Cleavage is "a capacity present in some rocks to break in certain directions more easily than in others," while fissility is a "structure in some rocks, by virtue of which they are already separated into parallel laminae in a state of nature. The term fissility thus complements cleavage, and the two are included under cleavage as ordinarily defined." (Van Hise).
Many unmodified igneous rocks have a marked cleavage, which is occasioned by the arrangement of the constituent mineral grains with their long axes parallel, or by a parallelism in the cleavage-planes of these minerals, or by both factors combined. In cleaved sedimentary rocks the cleavage-planes may coincide with the planes of stratification. Much more commonly, however, they intersect the latter at all possible angles, keeping a constant direction for long distances (parallel to the axes of the folds in which they occur), while the bedding-planes change with the dip from point to point. Ordinary roofing slate is one of the best possible examples of a cleaved rock and in beds of slate interstratified with other rocks, the cleavage is usually quite perfect in the former, absent or but partially developed in the latter.
It is very generally agreed among geologists that slaty cleavage is a result of compression; for, disregarding certain igneous masses it occurs only in rocks which show other evidences of having been subjected to compression. On the other hand, the mechanics of the problem are somewhat obscure and have given rise to differences of opinion. The most probable view seems to be that the cleavage-planes are developed at right angles to the compressing force, and are due to the arrangement of the constituent mineral particles of the rock with their longest diameters, their cleavage-planes, or both, in parallel directions. Further, that " this arrangement is caused, first and most important, by parallel development of new minerals ; second, by the flattening and parallel rotation of old and new mineral particles; and third, and of least importance, by the rotation into approximately parallel positions of random original particles." (Van Hise.) Fissility is also due to compression, the rocks giving way along the shearing-planes, which are inclined to the direction of the pressure.
Slaty cleavage is brought about in the softer rocks and fissility in the more rigid by similar compression.
A more advanced degree of metamorphism is characterized by the schistosity or foliation of the rocks, as is also true of contact metamorphism when such a rock as mica schist is formed. Schistosity or foliation is the arrangement of the component mineral particles of a rock into rudely parallel planes or undulating surfaces, in consequence of which the rock parts most readily along those planes or surfaces, and has a banded appearance. In the schistosity which is developed in the contact metamorphism of a sediment, the foliation appears to be determined by the stratification planes, but in regionally metamorphosed rocks this is generally not the case. Here the foliation, like cleavage and fissility, with which the former is connected by all grades of transition, as a rule, is independent of previous structures, and is determined by the direction of the compressing force. The intergradations between cleavage and fissility, on the one hand, and schistosity on the other, make it appear that all those structures are due to the same agency operating with different degrees of power under somewhat different circumstances.
The shearing and crushing of the rocks frequently change the component minerals into paramorphic forms, i.e. those which have the same chemical composition, but different crystal forms; for example, aragonite is thus converted into calcite and augite into hornblende. In the more complete stages of metamorphism an entire chemical reorganization is made, and new minerals are abundantly generated. Inasmuch as great areas of metamorphic rocks are almost invariably those which have been intensely and violently compressed, and moderately folded sedimentary rocks may sometimes be traced directly into intensely plicated metamorphic rocks, we are justified in concluding that the compression is the cause of the reconstruction, especially as the exceptions are more apparent than real. If this conclusion is well founded, it leads to the highly interesting and important generalization first clearly stated by President Van Hise, that the structures impressed on the stratified rocks after their first formation, folds, faults, thrusts, joints, cleavage, fissility, and foliation are all due to. lateral compression, acting with different degrees of intensity and at different depths, depth and overlying load being controlling factors of the first importance.
There is some difference of opinion as to the relative importance of contact and dynamic metamorphism, though it is not disputed that large areas may be metamorphosed by frequent and extensive igneous intrusions, nor that such intrusions may aid very materially in the transformations made by intense compression.