Page:The American Cyclopædia (1879) Volume XIV.djvu/386

 370 KOCKS tor is broad and deep, and there is a very active trade, particularly in the exportation of lime, made from immense quarries of limestone in the vicinity. There are about 80 kilns, era- ploying 1,000 men, and producing 1,200,000 barrels annually. Ship building is also carried on, and there are manufactories of boots and shoes, carriages, cooperage, edge tools, iron castings, harness, machinery, trunks, &c. The city has a fire department, water works, three hotels, two national banks with a joint capital of $250,000, a state bank with a capital of $50,000, a. savings bank with about $900,000 deposits, graded public schools, two weekly newspapers, and eight churches, viz. : Baptist (2), Congregational, Episcopal, Freewill Bap- tist, Methodist, Roman Catholic, and Univer- salist. Rockland was,et off from Thomaston and incorporated as a town, with the name of East Thomaston, in 1848. The name was changed in 1850, and in 1854 a city govern- ment was organized. ROCKS, in geology, the solid mineral masses which make up the earth's crust. These may be considered both geologically and minera- logically ; mineralogy is the natural history of all such bodies as do not belong to the organic, kingdoms of nature. In the geological investi- gation of rocks two questions arise : first, as to their structure and attitude and the mode of the.ir arrangement in the earth's crust, whether stratified or unstratified, whether occurring in beds, veins, or intruded masses; and second, their origin and mode of formation. The geognostical relations of rocks, and the dis- tinctions of crystalline and uncrystalline, of stratified and unstratified, of indigenous, ex- otic, and endogenous rocks, have been defined in the article GEOLOGY. Mineralogically rocks may be homogeneous or heterogeneous ; that is to say, they may consist of one or of two or more mineral species. Thus a pure white marble is made up entirely of calcite, a form of carbonate of lime, and quartzite consists of the mineral quartz, both homogeneous ; while granite is heterogeneous, consisting of a mix- ture of quartz and feldspar, sometimes with the addition of mica or of hornblende. (See GRANITE.) These component mineral species are sometimes so arranged as to show that the rock has resulted from an original crystal- lization, as in the case of granite or vitreous quartzite; and at other times, as in the case of sandstones and conglomerates, the aggre- gate is seen to be composed of the ruins of such rocks rearranged and cemented together. Hence the great distinction between original and derived rocks, the former including all exotic or eruptive rocks and all endogenous rocks or veinstones, as well as the crystalline indigenous rocks. Certain rocks owe their origin directly to the accumulations of organic structures; such are coal, which consists of vegetable remains, many limestones, which are made up of corals, shells, or encrinites, and certain silicious beds composed of the shields of diatoms. "We have thus a distinc- tion, which is sometimes made, of chemically, mechanically, and organically formed rocks. The chief mineral species of original rocks are calcite, dolomite, gypsum, anhydrite, rock salt, quartz, orthoclase, albite and the related tri- clinic feldspars, nepheline and certain zeolites, the micas, chlorites, talc, serpentine, olivine, pyroxene, hornblende, garnet, epidote, stauro- lite, kyanite, andalusite, tourmaline, graphite, magnetite, hematite, corundum, and pyrite. A few of these form rocks by themselves; others are essential ingredients of composite rocks ; while others occur as accessory though characteristic minerals in certain rock masses. The compound or heterogeneous rocks can be accurately defined only by describing the com- ponent minerals, their proportions and mode of arrangement, and the texture and structure of the mass. Arbitrary names have been given to certain types of composite rocks, but the student soon learns that there are many in- termediate varieties and admixtures which it is difficult to name or to classify. In de- scribing rock masses the geognostical distinc- tions of indigenous, exotic, and endogenous are to be disregarded, as in very many cases it is impossible from the study of a specimen to say to which division it belongs. Thus we have indigenous and endogenous crystalline limestones, and in the case of granitic rocks the characters of indigenous, endogenous, and exotic are often so similar that it is only by study of the rock in situ that it can be deter- mined to which class it belongs. The struc- ture of original rocks is not always crystalline ; some, like pearlstono and obsidian, being glass- like and amorphous. Others, though crys- talline, are so finely grained as to be compact, and are designated as crypto-crystalline. To rocks in which distinct crystals are imbedded in a compact or crypto-crystalline base the name of porphyry is given, and this is some- times extended to rocks in which the base en- closing the crystals is not compact. The terms gneissic and granitic, or gneissoid and grani- toid, are sometimes employed to designate rocks which, although unlike in composition, resemble gneiss or granite in structure and texture. The principal homogeneous original rocks are those composed of quartz, of car- bonate of lime, and of dolomite. Gypsum, the ores of iron, and occasionally certain sili- cates, such as labradorite, serpentine, talc, and chlorite, form by themselves considerable rock masses. The most important indigenous rocks are heterogeneous, and foremost among these may be named those essentially made up of quartz and orthoclase feldspar, constituting the granitic rocks, which generally include mica or hornblende as an accessory mineral. The indigenous banded rocks of this composition take the name of gneiss, and are either horn- blendic or micaceous, the latter passing into mica schist, so common with gneiss in the Montalban or White mountain series. By the