front

Front, in meteorology, zone of transition between adjacent air masses. If a cold air mass is advancing to replace a warmer one, their mutual boundary is termed a cold front; if the reverse, then the boundary is termed a warm front, whereas a stationary front indicates that no relative advance of either air mass is occurring. An occluded front is one in which a warm front has been completely undermined by cold air and is therefore positioned aloft. Since warmer air always overrides colder, denser air, the frontal boundary is sloped closer to the horizontal than the vertical. A mature cyclone usually involves all of the frontal types. The recognition of atmospheric fronts and their relative importance to weather forecasting came about only at the beginning of the 20th cent. as a result of publications by the meteorologists Vilhelm and Jakob Bjerknes.

Natural selection

Natural selection, process that results in adaptation of an organism to its environment by means of selectively reproducing changes in its genotype. Variations that increase an organism's chances of survival and procreation are preserved and multiplied from generation to generation at the expense of less advantageous variations. As proposed by Charles Darwin, natural selection is the mechanism by which evolution occurs. It may arise from differences in survival, fertility, rate of development, mating success, or any other aspect of the life cycle. Mutation, gene flow, and genetic drift, all of which are random processes, also alter gene abundance. Natural selection moderates the effects of these processes because it multiplies the incidence of beneficial mutations over generations and eliminates harmful ones, since the organisms that carry them leave few or no descendants.

Amber

Amber, fossilized tree resin. Amber can vary in color from yellow to red to green and blue. The best commercial amber is transparent, but some varieties are cloudy. To be called amber, the resin must be several million years old; recently hardened resins are called copals.

The tree species that produced amber are now extinct. They included cedars and other conifers and broadleaved trees. The most famous source of the world's amber is the Baltic coast of Germany. Amber is also found off the coasts of Sicily and England and in Myanmar (Burma). In the Western Hemisphere, there are rich deposits in the Dominican Republic, Mexico, and the state of New Jersey.

Amber is of interest both for its decorative value and for the ancient, once-living inclusions that it preserves. Capable of being highly polished, it is the oldest decorative substance known. It was familiar to Paleolithic peoples and to the Greeks and Romans, who used it extensively in jewelry. Pliny recounts several instances of its artistic uses. Amber is used in the manufacture of beads, amulets, mouthpieces, cigar and cigarette holders, pipes, and other small ornamental objects.

When rubbed with a cloth, amber becomes charged with static electricity; Thales was familiar with its electrical properties. When destructively distilled, amber yields acetic, butyric, valeric, and other acids; water; and hydrocarbons. Baltic amber also contains succinic acid and is often called succinite. An essential oil (amber oil) is obtained from amber.

Leaves, flowers, insects, and small animals are frequently found in amber. Older fossils trapped in this way often represent the sole specimen of an extinct species. An especially rich bed of amber in New Jersey has yielded over 100 previously unknown extinct Cretaceous species dating back as much as 94 million years. Because of amber's preservative qualities, the DNA of the specimens trapped inside is intact, affording scientists a unique opportunity to study the DNA of extinct species.

Sedimentary rock

Sedimentary rock, rock formed at or near the Earth's surface by the accumulation and lithification of fragments of preexisting rocks or by precipitation from solution at normal surface temperatures. Sedimentary rocks can be formed only where sediments are deposited long enough to become compacted and cemented into hard beds or strata. They are the most common rocks exposed on the Earth's surface but are only a minor constituent of the entire crust. Their defining characteristic is that they are formed in layers. Each layer has features that reflect the conditions during deposition, the nature of the source material (and, often, the organisms present), and the means of transport. 

Fossil

Fossil, remains or imprints of plants or animals preserved from prehistoric times by the operation of natural conditions. Fossils are found in sedimentary rock, asphalt deposits, and coal and sometimes in amber and certain other materials. The scientific study of fossils is paleontology. Not until c.1800 were fossils generally recognized as the remains of living things of the past and accepted as an invaluable record of the earth's history.

The Formation of Fossils

Conditions conducive to the formation of fossils include quick burial in moist sediment or other material that tends to prevent weathering and to exclude oxygen and bacteria, thereby preventing decay. Shells and bones embedded in sediment in past geologic time, under conditions suitable for preservation, left exact reproductions of both external and internal structures. Skeletal remains have been preserved as a result of the engulfment of an animal's body in ancient asphalt pits, bogs, and quicksand. At Rancho La Brea, near Los Angeles, Calif., asphalt deposits have yielded a rich variety of skeletons of birds and mammals. Some fossils have been found buried in volcanic ash; such fossil deposits exist in the Cenozoic rocks of the W United States.