Handbook of Paleontology 73 



and gases. Metamorphic rocks are most character- 

 istic of strongly deformed areas and are also found 

 in areas of igneous intrusion (contact metamorphism). 

 The term regional metamorphism is used in contrast to 

 contact metamorphism which is often quite local. Cleav- 

 age is developed through metamorphism by a parallel 

 orientation or rearrangement of minerals. It is particu- 

 larly well developed in the finer-grained rocks such as 

 shales, which are converted into slates with slaty cleav- 

 age, as discussed under sedimentary rocks (page 56). 

 Cleavage developed in a zone of fracture is known as 

 fracture cleavage. When a rock splits easily but not as 

 regularly as in slaty cleavage the structure is known as 

 schistosity and is characteristic of schists. Sometimes, 

 as in gneisses, parallel linear structure is developed by 

 the formation of crystals with their lengths parallel to 

 the line of least force or stress. Original structures in 

 rocks, such as fossils, pebbles, cross-bedding, ripple marks 

 etc., are distorted through metamorphism. Fossils may 

 be so distorted that they are unrecognizable and may even 

 disappear entirely. 



Metamorphism of sedimentary rocks converts con- 

 glomerates into gneisses and schists ; sandstones into 

 quartzites and schists ; shales into slates and schists ; 

 limestones into marbles or, if impure, into schists. 

 Among igneous rocks, coarse-grained feldspathic rocks, 

 such as granite, become gneisses ; fine-grained feldspathic 

 rocks, slates and schists; ferromagnesian rocks, various 

 kinds of schists and serpentine. Gneiss is not only the 

 name of a rock but also expresses texture, and when 

 used alone means a rock composed, as granite is, of 

 quartz, feldspar and mica with foliated structure. In 

 granite-gneiss, syenite-gneiss and diorite-gneiss the first 



