POST-PLIOCENE PERIOD CONTINUED.-GLACIAL EPOCH.

Geographical distribution, form, and characters of glacial drift-Fundamental rocks, polished, grooved, and scratched-Abrading and striating action of glaciersMoraines, erratic blocks, and "Roches Moutonnées "-Alpine blocks on the Jura -Colossal size of ancient Swiss glaciers-Continental ice of Greenland-Ancient centres of the dispersion of erratics-Transportation of drift by floating icebergs-Bed of the sea furrowed and polished by the running aground of floating ice-islands-How to distinguish glacial drift of submarine from that of terrestrial origin.

AMONG the different kinds of alluvium described in Chapter VII., a passing allusion was made (page 80) to the "boulder formation" and to its origin as probably connected with the agency of glaciers and floating ice. This formation, to which many names, such as "diluvium," "northern drift," "boulder clay," and "glacial deposits" have been given, is abundant in Europe north of the 50th, and in North America north of the 40th parallel of latitude. It is wanting in the warmer and equatorial regions, and reappears when we examine the lands which lie south of the 40th and 50th parallels in the Southern Hemisphere; as, for example, in Patagonia, Terra del Fuego, and New Zealand. It consists of sand and clay, sometimes stratified, but often wholly devoid of stratification for a depth of 50, 100, or even a greater number of feet. To this unstratified form of

the deposit the name of till has long been applied in Scotland. It generally contains a mixture of angular and rounded fragments of rock, some of large size, having occasionally one or more of their sides flattened and smoothed, or even highly polished. The smoothed surfaces usually exhibit many scratches parallel to each other, one set of which often crosses an older set. The till is almost everywhere wholly devoid of organic remains, except those washed into it from older formations, though in some places it contains marine shells of arctic species, many of them in a fragmentary state. The bulk of the till has usually been derived from the grinding down into mud of rocks in the immediate neighborhood, so that it is red in a region of Red Sandstone, as in Strathmore in Forfarshire; gray or black in a district of coal and coal-shale, as around Edinburgh; and white in a chalk country, as in parts of Norfolk and Denmark. The stony fragments dispersed irregularly through the till usually belong, especially in mountainous countries, to rocks found in some parts of the same hydrographical basin; but there are regions where the whole of the boulder clay has come from a distance, and huge blocks, or erratics," as they have been called, many feet in diameter, have not unfrequently travelled hundreds of miles from their point of departure, or from the parent rocks from which they have evidently been detached. These are commonly angular, and have often one or more of their sides polished and furrowed.

The fundamental rock on which the boulder formation reposes, if it consists of granite, gneiss, marble, or other hard stone, capable of permanently retaining any superficial markings which may have been imprinted upon it, is usually smoothed or polished, like the erratics above described; and exhibits parallel striæ and furrows having a determinate direction. This direction, both in Europe and North America, agrees generally in a marked manner with the course taken by the erratic blocks in the same district.

The boulder clay, when it was first studied, seemed in many of its characters so singular and anomalous, that geologists despaired of ever being able to interpret the phenomena by reference to causes now in diurnal action. In those exceptional cases, where marine shells of the same date as the boulder clay were found, nearly all of them were recognised as living species-a fact conspiring with the superficial position of the drift to indicate a comparatively modern origin. The recentness of the date caused the enigma to appear only the more perplexing, and strengthened the belief that the phenomena were the results of forces distinct both in kind and energy from those now operating in the ordinary course of nature. Notions of this kind were calculated to retard the progress of science, by diverting attention from such every-day operations as were capable of producing analogous effects.

The term "diluvium" was for a time the most popular name of the boulder formation, because it was referred by many to the deluge of

Noah, while others retained the name as expressive of their opinion that a series of diluvial waves raised by hurricanes and storms, or by earthquakes, or by the sudden upheaval of land from the bed of the sea, had swept over the continents, carrying with them vast masses of mud and heavy stones, and forcing these stores over rocky surfaces so as to polish and imprint upon them long furrows and striæ.

But geologists were not long in seeing that the boulder formation was characteristic of high latitudes, and that on the whole the size and number of erratic blocks increases as we travel toward the arctic regions. They could not fail to be struck with the contrast which the countries bordering the Baltic presented when compared with those surrounding the Mediterranean. The multitude of travelled blocks and striated rocks in the one region, and the absence of such appearances in the other, were too obvious to be overlooked. Even the great development of the boulder formation, with large erratics so far south as the Alps, offered an exception to the general rule favorable to the hypothesis that there was some intimate connection between it and accumulations of snow and ice.

Abrading, polishing, scouring and transporting power of glaciers.It is well known that those parts of the Alps which rise to heights exceeding 8500 feet above the level of the sea are covered with perpetual snow. This snow, as it receives annual additions, would increase indefinitely in altitude were not its accumulation checked by the constant descent of a large portion of it by gravitation. As it glides slowly down the principal valleys flanking the highest mountains, it becomes converted into solid ice, and forms what are termed glaciers, or rivers of ice, the lower extremities of which, when they descend into warmer regions, melt and give rise to torrents of water. On the borders of every glacier are seen on either side mounds, or taluses of rubbish, consisting of angular fragments of rock, with large heaps of sand and mud. At certain distances from each side, and often in the centre, ridges composed of similar debris from three to twelve feet in height, are observable. Each of these has originated, like the lateral mounds, in the form of a talus accumulated at the foot of a steep slope or precipice. Frost, rain, lightning, and avalanches of snow are constantly detaching fragments of rock and soil which fall or roll down to the bottom of such precipices. If the base of the heap of loose materials were washed by a river, it would soon be undermined and swept away, but when this fallen matter reaches the edge of a glacier, which is always moving onward night and day at the rate of several inches, or sometimes a foot or two in twenty-four hours, the whole talus becomes locomotive, and is changed into a long stream of blocks and earthy matter, fringing the glacier on both sides, and constituting what are called lateral moraines. As often as glaciers are conflu ent, the right lateral moraine of one blends with the left moraine of the other, and both are then carried down in the middle of the mass

of ice produced by the union of the two glaciers, forming what is called a medial moraine. The number and position of these moraines will depend on the number and size of the tributary glaciers which join the main one. By such machinery, not only small stones and earth, but erratic blocks of the largest size, are carried down from the mountains to the lower valleys and plains, performing a journey of twenty or thirty miles in the course of several centuries, and usually retaining their edges sharp and unworn to the last.

When the glacier passes over uneven ground, it becomes rent, and traversed by broad and deep transverse fissures, into which portions of the lateral or medial moraines are precipitated. Rills of water also, derived from the liquefaction of the ice by the sun's rays in summer, run over the surface of the glacier until, arriving at one of these fissures, they cascade into it. From this source, as well as from springs, which must occasionally break out under the glacier, are derived torrents which flow under the ice in tunnels, where the angular stones which have fallen to the bottom through the fissures often become rounded, as in the ordinary bed of a river. Other blocks and pebbles, being fixed in the ice, and firmly frozen into it, are pushed along the bottom of the glacier, abrading, polishing, and grooving the rocky floor below, while each stone is reciprocally flattened, polished, and striated on its lower side. As the forces of downward pressure and onward propulsion are enormous, each small grain of sand, if it consists of quartz or some hard mineral, scratches and polishes the surface, whether of the underlying rock or of the boulder which impinges on it, as a diamond cuts glass or as emery powder polishes steel. The striæ which are made, and the deep grooves which are scooped out by this action, are rectilinear and parallel to an extent never seen in those produced on loose stones or rocks, where shingle is hurried along by a torrent, or by the waves on a sea-beach.

As water is always flowing under some parts of a glacier, and much melting and regelation are going on in different places, stones are liable to change their position, in which case a second set of stria and furrows may be imprinted in a new direction, or another side of the stone becomes, in its turn, flattened, striated, and polished. In like manner the solid rock underneath the glacier may exhibit scratches and grooves in more than one direction. The furrows will, most of them, coincide with the general course of the valley; but as the ice in different seasons varies in quantity, the direction of its motion at any given point is not uniform, so that the grooves and scratches will also vary, one set often intersecting another.

When a Swiss glacier, laden with mud and stones, descends so far as to reach a region about 3500 feet above the level of the sea, the warmth of the air is such that it melts rapidly in summer, and in spite of the downward movement of the mass, it can advance no farther. Its precise limits are variable from year to year, and still more so from century to century; one example being on record of a recession of

half a mile in a single year. We also learn from M. Venetz, that whereas, between the eleventh and fifteenth centuries, all the Alpine glaciers were less advanced than now, they began in the seventeenth

Limestone polished, furrowed, and scratched by the glacier of Rosenlaui, in Switzerland. (Agassiz.)

a a. White streaks or scratches, caused by small grains of flint frozen into the ice.
b b. Furrows.

and eighteenth centuries to push forward, so as to cover roads formerly open, and to overwhelm forests of ancient growth.

These oscillations enable the geologist to note the marks which a glacier leaves behind it as it retrogrades; and among these the most prominent is the terminal moraine, which is a confused heap of unstratified rubbish, like the till before described; all the mud, sand, and pieces of rock, with which the glacier was loaded, having been slowly deposited in the same spot where no running water interfered to sort them, by carrying the smaller and lighter particles and stones farther than the bigger and heavier ones. These terminal moraines often cross the valley in the form of transverse mounds, more or less divided into separate masses or hillocks by the action of the torrent which flows out from the end of the glacier. Such transverse barriers were formerly pointed out by Saussure, below the glacier of the Rhone, as proving how far it had once transgressed its present boundaries. On these moraines we see many large angular fragments, which, having been carried along on the surface of the ice, have not had their edges worn off by friction; there are also many boulders, of various sizes, which have been rounded; some, as before stated, by the power of water beneath the glacier, others by the mechanical force of the ice which has pushed them against each other, or against the rocks flanking the valley.

As the terminal moraines are the most prominent of all the monu