ISOSTATIC CORRECTION 



isodynamic — A line connecting points of equal 

 magnitude of any force. (5) 



isogal — A contour line of equal gravity values on 

 the surface of the earth. 



isogonic line — In the study of terrestrial mag- 

 netism, a line drawn through all points on the 

 earth's surface having the same magnetic 

 declination; not to be confused witla magnetic 

 meridian. Tlie particular isogonic line drawn 

 thi-ougli all points having zero declination ia 

 called the agonic line. (5) 



isogram — (or isollne). See isopleth. 



isohaline — 1. Of equal or constant salinity. 



2. A line on a chart connecting all points of 

 equal salinity ; an isopleth of salinity. 



(5) 



isonyte — 1. A line drawn through geographical 

 points recording equal amounts of precipitation 

 during a given time period or for a particular 

 storm. (5) 



2. A line which connects places with equal 

 average total preciptation. 



isoline — See isopleth. 



isomegathy — A chart showing the distribution of 

 sediments in terms of median grain size. 

 Isomegatliies are lines connecting points of 

 equal median grain size. (2) 



isomer — 1. An element, compound, or atomic 

 nucleus having the same atomic number and 

 mass number but differing in one or more prop- 

 erties, as is the rate of radioactive decay. 



One of several nuclides having the same num- 

 ber of neutrons and protons but capable of 

 existing, for a measurable time, in different 

 quantum states with different energies and 

 radioactive properties. Commonly, the isomer 

 of higher energy decays to one with lower 

 energy by the process of isomeric transition. 

 (70) 



2. Literally, equal parts. A British term for 

 a line on a chart along which an equal percent- 

 age of the annual precipitation falls in a given 

 month or season. 



isopach — (or isopachous^ isapachyte). A con- 

 tour line on a chart drawn through points of 

 equal thickness of a sedimentary layer. 



isopachous — See isopach. 



isopaehyte — A British term for isopach. 



isopag — A line connecting points where ice is 

 present for the same number of days per year. 



isopectic — A line connecting points at which ice 

 begins to form at the same time of the winter. 

 A line connecting points at which ice melts at the 

 same time of the spring is called an isotac. 



isopiestic — A term denoting equal or constant 



pressure. 

 isopleth — A line of equal or constant value of a 



given quantity, with respect to either space or 



time. 



isopod — One of an order (Isopoda) of generally 

 flattened crustaceans. They are mostly scaven- 

 gers. Several members burrow into wood. See 

 marine borers. 



isopor — A line sometimes found on magnetic 

 charts showing points of equal annual change. 

 (28) _ 



isopycnic — 1. Of equal or constant density, with 

 respect to either space or time ; equivalent to an 

 isostere. 



2. A line on a chart connecting all points of 

 equal or constant density ; an isopleth or density. 



3. Short form for isopycnic line, 

 isopycnic line — A line of equal or constant den- 

 sity. It is equivalent to an isostere. (5) 



isopycnic surface — A surface of constant density. 

 A particle of water of a certain density moves 

 along an isopycnic surface or, if forced away 

 from this surface, will seek to return to it. 



isostasy — A condition of approximate equilibrium 

 in the outer part of the earth, such that 

 the gravitational effect of masses extending 

 above the surface of the geoid in continental 

 areas is approximately counterbalanced by a 

 deficiency of density in the material beneath 

 those masses, while the effect of deficiency of 

 density in ocean waters is counterbalanced by an 

 excess of density of the material under the 

 oceans. 



isostatic adjustment — The process of restoring 

 and maintaining that condition of equilibrium 

 in the so-called crust of the earth which is known 

 as isostasy. 



The distribution of material in the outer part 

 of the earth is undergoing continual change by 

 the operation of erosion, sedimentation, and 

 other natural forces. The unbalanced condition 

 which would naturally result from such disturb- 

 ing processes is offset by the movement of ma- 

 terial at considerable depths below the surface 

 of the earth. (37) 



isostatic anomaly — The difference between an 

 observed value of gravity and a theoretical 

 value at the point of observation which has been 

 corrected for elevation of the station above the 

 geoid and for the effect of topography over the 

 whole earth and for its isostatic compensation. 

 (37) 



isostatic compensation — The departure from 

 normal density of material in the lower part of a 

 column of the earth's crust which balances (com- 

 pensates) landmasses (topography) above sea 

 level and deficiency of mass in ocean waters, and 

 produces the condition of approximate equilib- 

 rium of the earth's crust known as isostasy. 

 (37) 



isostatic correction — The adjustment made to 

 values of gravity or to deflections of the ver- 

 tical observed at a point to take account of the 

 assumed mass deficiency under topographic fea- 

 tures for which a topographic correction is also 

 made. 



89 



