greenstone is a common rock. The range of mountains on the west side of the Connecticut, reaching from NewHaven to Northampton, is of this rock. In some places, their height is several hundred feet. These rocks, as will be seen in another place, are undoubtedly of volcanic origin, having been elevated to their present situation through fissures, by the force of subterranean fire. Lava. This term comes from the Gothic, and signifies to run, in reference to the flowing of volcanic matter. The products of volcanic mountains often present very different appearances, and hence have received several names, as volcanic slags, volcanic enamel, cellular lava, compact lava, pumice, &c. But in general terms, all the liquified products of volcanoes are called lava, and for the purposes of elementary geology, this definition is perhaps sufficient. The colors of lava are most commonly yellowish, or greenish gray sometimes running into sulphur yellow, and greyish black. Some are compact, while others are full of small pores, and others are fibrous with a silky lustre; but all the different kinds run into each other, so that it is often difficult to make distinctions between them. DILUVIUM. Diluvia, or diluvial deposites, are generally supposed to have been formed during the general deluge. They consist of sand, pebbles, and blocks, or fragments of various kinds of rocks, not generally existing in the districts where these deposites are found at the present day, and hence they must have been transported from a distance. In many instances, the diluvial rocks appear to have been moved from great distances, their dimensions and situations at the same time indicating a water power of much greater force, than any which has been described, except the Noachian deluge, and it is therefore considered reasonable to attribute these effects to that cause.-See Deluge. ALLUVION. Alluvia, or Alluvial deposites are such accumulations of sand, mud, and soil, together with fragments of wood, as are constantly forming at the present day, by the currents of rivers and brooks, or by the rain which falls on hills and mountains. These are formed by causes now constantly operating, and we have shown that considerable changes have been wrought on the earth by such causes. We have now given a short account of each formation, and species of rock which compose the great bulk of the earth. There are however, several rocks described in more extended treatises on this subject which we have omitted, and which occasionally form considerable hills, or underlay certain districts of country. This deficiency with respect to names, will be supplied by the following view of M. Boue's classification of rocks, corrected and extended by Dr. Ure, of Glasgow. This contains the names of all the known members of each class, and by it the student will be enabled to observe the synonomous terms, with the classification we have employed. Order VI.-Iron Sand and Chlorite Chalk. Green Sand. Order VII.-Chalk. Chalk Marl, Class V.-SUPERIOR OR TERTIARY Rocks. Order I.-London, Paris and Plastic Clay, sins. Sand, lignite and salt water shells. Order II.-First Tertiary Blue London Clay, Limestone. Chloritic Limestone. Order III.-First Local brack- Marls, Order IV.-Second Tertiary Marls, Limestone. Class VI.-VOLCANIC PRODUCTS. Burh-stones of Paris basin, and Isle of Wight. Order I.-Basaltic Rocks. Basalt, Porphyry. Order II.-Lava Lava, It will be observed in this classification, that the same formation or kind of rock sometimes occurs more than once, or is arranged under several different classes or orders. Thus limestone is sometimes primitive, at others, transition, secondary, or tertiary; and clay-slate and sandstone are sometimes associated with one formation, and sometimes with another. When, therefore, a rock, under the same name, is supposed by its associations, to have been formed at different periods, it is classed severally with those of its own age. Thus limestone is of all ages, and consequently belongs to all the classes, except the volcanic. The same is more or less the case with sandstone and clay-slate, and several others. COMPARATIVE AGES OF ROCKS. We have already noticed, under the descriptions of the different formations, their relative ages, but a recapitulation is required in order to bring this subject distinctly before the reader. It requires no arguments to show that the lowest formations must be the oldest, since these must have been deposited before those which lie above, or upon them. It is true that a mountain of granite, when shaken, or uplifted by an earthquake, may fall and spread its ruins on the plain below, but such an occurrence would readily be detected, since the situation of its fragments would show that this was not an original and undisturbed formation. Granite and its associates, besides being placed lowest in the order of position, are, as we have already seen, entirely destitute of organic remains. It ought, however, to be noticed that Dr. Macculloch, in a single instance, in one of the Hebrides, observed gneiss overlaying a bed of limestone, which contained bivalve shells. But the ex |