METAMORPHISM 293 



Beds 1. 1" prat, limiitr, and bituminous coal are converted into anthra- 

 cite I>y thr driving off of the volatile hydrocarbons. If the process 

 ^ors to thr extreme, graphite is the result. 



Metamorphism by pressure. When rocks made up of clastic 

 particles are compressed in one direction, and are relatively free to 



Fii, r . 284. Figure showing the elongation of pebbles under pressure. Carbon- 

 itVmus formation, Newport, R. I. (Walcott, U. S. Geol. Surv.) 



expand in others, the particles which are already elongated tend to 

 turn so that their longer axes are at right angles to the direction of 

 pressure, and all particles, whether elongate or not, are more or less 

 flattened at right angles to the direction of stress. This is readily 

 seen where the particles are large (Fig. 284). As a result of the turn- 

 ing (or orientation) and flattening of their particles, rocks so affected 

 split more readily between the elongate and flattened particles than 

 across them. In other words, the rocks cleave along planes normal 

 to the direction of compression. The structure thus induced is 

 known as slaty structure (Fig. 285), and is illustrated by roolm.^- 

 slate, which was originally a mud, later a shale, and finally assumed 

 the slaty condition under strong compression. In some cases the 

 original bedding may still be seen running across the cleavage planes 



