810 A TREATISE ON METAMORPHISM. 



schistose, or gneissose rocks, in that they are massive and evenly granular, 

 although the mineral particles commonly show other strain effects men- 

 tioned below. The explanation of the general lack of slaty and schistose 

 structures is believed to lie largely in the fact that the process of recrys- 

 tallization under favorable conditions goes on almost pari passu with the 

 deformation, and thus at any given moment the minerals lack but little of 

 adjustment to their environment. In so far as the calcite and dolomite are 

 left with residual strain there is a tendency to recrystallize. Their mobility 

 is such that this tendency often prevails, and the minerals are largely 

 released from strain. But in many instances release from strain is not per- 

 fect, for when thin sections are examined in many of the apparently com- 

 pletely crystalline marbles strain effects remain. Cataclastic structures 

 occur in variable amounts; strain shadows are usual; twinning bands are 

 very abundant, often are exceedingly' fine, and 

 frequently the lamella? are curved, in such in- 

 stances showing conclusively the existence of 

 residual strain effects. And it is not always true 

 that during the development of marble the pro- 

 cess of crystallization so nearly keeps pace with 

 movement as to prevent the formation of a sec- 

 -^ketchr,f ovai irregular grams onc i ary structure. In such cases the marbles 



of calcite with longer diameters par- */ 



allel. From schistose marble. Talla- ma y h ave a well-developed cleavag'e. This ' is 



dega Mountain, Alabama. After Leith. J 1 o 



well illustrated in some of the pre-Cambrian 

 cleavable marbles of the Laurentian Mountains, by the cleavable Cambro- 

 Silurian marbles of western Massachusetts, described by Dale," and in the 

 marbles of Alabama at Talladega Mountain. (See fig. 22 and PI. V, A) 

 At this latter locality the marble cleaves as evenly and smoothly as a slate. 

 Where marbles have a cleavage it is found to conform to the cleavage of 

 the surrounding rock. The cleavage is found to be whollv due to a 

 dimensional arrangement of the mineral particles. (See Chapter VIII, 

 p. 760.) The fact that the cleavage of the mineral particles has nothing 

 to do with the cleavage of the rock is explained by the rhombohedral 

 cleavage of calcite resulting in fracture with equal ease in three directions. 

 In this respect calcite and dolomite contrast strongly with the minerals 



"Pumpelly, Raphael, Wolff, J. E., and Dale, T. Nelson, Geology of the Green Mountains in 

 Massachusetts: Mon. IT. S. Geol. Survey, vol. 23, 1894, pp. 181-182. 



