_Limestone._--A sedimentary rock consisting essentially of carbonate of lime which generally represents accumulation of shells of organisms, but in some cases precipitates from solution. Often impure.
_Loess_ (pronounced lurse with the r obscure).--A fine homogeneous silt or loam showing usually no division into layers and forming thick and extensive deposits in the Mississippi Valley and in China. It is generally regarded as in part at least a deposit of wind-blown dust.
_Marble._--A crystalline limestone, usually a metamorphic rock, the limestone having been altered by heat, pressure, and moisture within the earth.
_Meander._--To flow in serpentine curves. A loop in a stream. Most streams in flowing across plains develop meanders.
_Mesa._--A flat-topped hill or mountain left isolated during the general erosion or cutting down of a region.
_Mesozoic._--Next to the present era of geologic time. _ Metamorphic Rock._--Any igneous or sedimentary rock which has undergone metamorphism, that is notable alteration from its original condition. (See Metamorphism.)
_Metamorphism._--Any change in rocks effected in the earth by heat, pressure, solutions, or gases. A common cause of the metamorphism of rocks is the intrusion into them of igneous rocks. Rocks that have been so changed are termed metamorphic. Marble, for example, is metamorphosed limestone.
_Mineral._--An inorganic substance of definite chemical composition found ready made in nature, e.g. calcite, quartz.
_Mississippian._--A period of the Paleozoic era of geologic time--in order of age, the third from the last of the era.
_Moraine._--Glacial drift carried on, within, or under a glacier and deposited at the end, along the sides, or under the glacier.
_Oil-pool._--An accumulation or body of oil in sedimentary rock that yields petroleum on drilling. The oil occurs in the pores of the rock and is not a pool or pond in the ordinary sense of these words.
_Ordovician._--Next to the earliest period of the Paleozoic era of geologic time.
_Ore._--A metal-bearing mineral or rock of sufficient value to be mined.
_Outcrop._--That part of a rock formation which appears at the surface. The appearance of a rock at the surface or its projection above the soil. Often called an exposure.