WorldWide Drilling Resource

Frequently-Used Geological Terms Part 55 Glossary Adapted from the Dictionary of Geological Terms M Mylonite - Acompact, chert-like rock with a streaky or banded structure, produced by the extreme granulation and shearing of rocks which have been pulverized and rolled during overthrusting or intense dynamic metamorphism. Mylonite may also be described as a microbeccia with flow texture. Myrmekite - An intergrowth of plagioclase feldspar (usually oligoclase) and quartz, generally replacing potassium feldspar, formed during the later stages of consolidation in an igneous rock or during a subsequent period of plutonic activity. The quartz occurs as blobs or wormlike shapes within the feldspar. N Nahcolite - A white monoclinic mineral. Nappe - A sheetlike, allochthonous rock unit which has moved in a predominantly horizontal surface. The mechanism may be thrust faulting, recumbent folding, or gravity sliding. Natural Coke - Coal which has been nat- urally carbonized by contact with or prox- imity to, an igneous intrusion, or by natural combustion. Nebraskan - Pertaining to the classical first glacial stage of the Pleistocene Epoch in North America, followed by the Aftonian interglacial stage. Needle - A needle-shaped mineral crystal. Also refers to a pointed, elevated, and de- tached mass of rock formed by erosion. Nematoblastic - Pertaining to a type of even-grained texture in metamorphic rocks resulting from development during re- crystallization of slender parallel pris- matic crystals of actinolite. Neolithic - In archaeology, the last division of the Stone Age, characterized by the development of agriculture and domes- tication of farm animals. Neomineralization - Chemical inter- change within a rock resulting in the al- teration of its mineral components and production of new minerals. It is a type of recrystallization. Look for more terms next month! Photo of mylonitic and cataclastic rock in the southeastern San Bernardino Mountains in southern California, courtesy of the U.S. Geological Survey. 27 WorldWide Drilling Resource ® NOVEMBER 2016