ROCKWALL admixture in the hornblendio varieties of a triclinic feldspar (albite, oligoclase, or labra- dorite), which finally replaces the orthoclase, and the disappearance of the quartz, we get the rock known as diorite ; and the substitu- tion in such a rock of some form of pyroxene for hornblende produces what are known as dolerite and diabase. To these last named three rocks belong most of the so-called traps, basalts, and greenstones, which are generally intrusive -or exotic rocks, although indigenous rocks, composed of triclinic feldspars with a greater or less admixture of hornblende or of pyroxene, are abundant. Chlorite often ac- companies the hornblende of these rocks, or replaces it, especially in the Green mountain or Huronian series, where such rocks are as- sociated with strata in which a soft hydrous mica prevails, forming the so-called talcose slates, which seldom contain talc. Rocks com- posed chiefly of labradorite, sometimes with- out admixture, and at other times with small portions of hypersthene or of pyroxene, pre- dominate in the Norian series. Under the title VOLCANO will be considered the history of volcanic rocks, and there and under WATER will be discussed the chemical agencies which have produced the various rocks, the genesis of which cannot be well understood without a reference to the chemico-geological effects of fire and of water. Much has been done of late in the minute study of the crystalline rocks, and here the microscope has been used with much success. Among the best works on the subject of lithology are those of Bern- hard von Cotta, Senft, and especially Zirkel. ROCKWALL, a N. E. county of Texas, wa- tered by affluents of Trinity and Sabine rivers, formed since the census of 1870. It consists chiefly of undulating prairies, and has a good soil. Capital, Rockwall. ROCKY MOUNTAIN LOCUST, an insect belong- ing to the same family (locustidm of West- wood, acrididce of later authors) as the locusts of the old world and of 'Scripture. It is the only species in this country, E. of the Rocky mountains, that has the same migratory habit and great power for harm which character- izes those whose ravages are described by the prophet Joel, and which have figured so large- ly in the history of southern European and Asiatic nations. A species often complained of on the Pacific slope is probably the same, or a variety of the same. The Rocky mountain locust, first specifically characterized by Prof. Cyrus Thomas, in his "Acrididse of North America" (1873), as caloptenus spretus, is pop- ularly known as the grasshopper, a term loose- ly applied to most large hopping insects. Du- ring 18.73, 1874, and 1875 this insect attracted unusual attention, and in the unprecedented amount of injury and suffering which it en- tailed on the farmers of the west it proved a national calamity. In 1873 Minnesota and Iowa were sorely scourged by it in their west- ern counties, and had to appeal to the nation ROCKY MOUNTAIN LOCUST 371 for assistance to relieve the consequent suf- fering; in 1874 Nebraska and Kansas suffered to such an extent that the efforts of the state authorities and the contributions of people from all parts of the Union were insufficient to prevent a vast amount of distress ; while in FIG. 1. Rocky Mountain Locust (Caloptenus spretus). the spring of 1875 parts of Missouri and Kan- sas were again terribly smitten. It has been estimated that $50,000,000 would not cover the loss occasioned to the country by this in- sect during these three years. The species measures on an average about 1 i n - from the head to the tip of the closed wings, and the wings extend about one third their length be- yond the tip of the abdomen. The color is variable, but the more common specimens are yellowish white beneath; glaucous across the breast and about the mouth parts ; pale bluish glaucous, often with shades of purple, on the sides of the head and thorax and on the front of the face; olive-brown on the top of head and thorax; pale beneath, more or less blu- ish above and marked with black, especially toward base, on the abdomen. The front wings have the ground color pale grayish yellow, inclining to green, and their spots and veins brown ; the hind wings, except a yellowish or brownish shade at apex and along the front edge and a green tint at base, are transparent and colorless, with the veins brown. The front and middle legs are yellowish. The hind legs have the thighs striped with pale glaucous and reddish on the outside and upper half of inside, with four broad black or dusky marks on the upper edge, the terminal one extending be- neath around the knee. The shanks are coral- red with black spines ; the feet somewhat paler, with .black claws; antennre pale yellow ; palpi tipped with black. - In the dead specimens all these colors become more dingy and yellow. It very closely resembles, and is often con- founded with, the red-legged locust (caloptenus femur-nibrum, De Geer), a species common to the whole central portion of the continent FIG. 2. Red-legged Locust (Caloptenus femur-rubrum). from the Atlantic to the Rocky mountains, which, though capable of short flights, never commits the same havoc. This last is, on an average, smaller, darker, with shorter and less conspicuously spotted wings (seldom extend- ing more than one sixth of their length fee-
