beach of Newport. South of this point we have two considerable ponds, Almy's and Lilly pond; both of these and the marshy pond just to the west of the latter are glacial excavations. The last is closing by the accumulation of vegetable matter, its waters having become shallow enough to bear an abundant vegetation, which is rapidly converting it into a peat bog. The island is singularly free from perfectly level lowlands, such as are formed by the filling up of old ponds, and the few sheets of water which remain are not disappearing with the rapidity usual in New England.

The process of occlusion in these ponds can be studied to advantage in the marshes between the Lilly pond and Bateman's. The vegetation, consisting of a variety of marsh plants, begins its growth at the shore where there is a sand beach and bold water, but it may be a long time before its foothold can be made good. Some accident such as a landslide or a fallen tree makes a little shelter, so that the plants get a hold in the water. In a short time they make a bed a foot or more in thickness; their roots go so deep that soon they can stand a considerable beat of the wave. The commotions in the water tear away and distribute over the floor of the pond a part of the mass, but it continues to grow and gains on the clear space, often at the rate of several inches a year. Where the circumstances are favorable, we find the mat of plants extending from the shore over the pond, without touching its floor, sometimes for only a few feet, again, at times, covering many acres with its growth. The accumulation of sediment under these conditions takes place in a singular fashion; the mat of vegetation floats upon the surface of the pond, and sinks, deeper and deeper as it grows until, finally, it rests upon the bottom. It then continues to grow until it has gotten itself so far above the water, that the vegetation no longer has the necessary amount of water.*

The topography of the island, as a whole, is remarkable for the small extent to which it expresses the structure of the rock below.

* This process of growth makes the peat bogs of all countries the natural repositories of the implements of man: where a stone tool remained on the surface it was likely to be found again and again, and used until its very chips were worn away. Falling into a bog, slipping through some crevice in its top crust, it sank to a secure hiding place. The fact that implements can penetrate so deeply into bogs makes them very untrustworthy recorders of time. An implement of the stone age may get buried in the solid matter two feet from the surface, while a modern piece of work falling into a more open spot may sink far below it. When the bog comes to be excavated the deeper buried object is naturally, but mistakenly, taken for the older.

When we come to study the disposition of the underlying rock we shall be struck by the great amount of perturbation the beds have suffered; they are thrown into real mountain irregularities. Few parts of the Alleghanies are more seriously disturbed, but the surface preserves no semblance of mountain forms. It does not differ from the shape which horizontally lying rock would give. If the character of the surface had been given by water action, then we should have had something very different from this. The valleys would then have mapped for us the attitude of the strata and the elevations would have been much greater and differently disposed from what they are now. But under the sheet of ice these details of structure lose their value; the ice cannot turn and twist as the water does, only a slight deflection even under the most important resistance can be permitted. The result necessarily is that the glacial stream wears away without much reference to the disposition of the beds beneath the surface. This is a point of more than local importance; it may serve often as a general guide to the determination of the question whether any particular country shows the result of glaciation. When the topography of any region does not express the structure of its underlying materials, in the fashion common to all surfaces of purely aqueous erosion, then there is reason to suspect the action of ice. This suspicion may amount to a certainty wherein the whole topography has been created with the great independence of internal conditions which it here exhibits.

There is a general absence of sand and pebbles in the beaches about this island; this is, probably, primarily due to the fact that drift has not been enough washed over by the sea to remove the arenaceous matter. The sand mass of Nantasket or Lynn beaches alone much exceeds all that exists in the spits and beaches about Narraganset Bay. The small extent to which the drift has been robbed of its sands is due to the original slight submergence of this district; only about ten to twenty feet of emergence is indicated, so the region has not been so much washed over as the region more to the north. There are great bodies of sand on the shores to the north and south; its comparative absence here is to be explained by the want of recent great movements of the land, and the deep water which prevents its movement along the shore from the east or west. As we go south of this region we come into the area of positive subsidence since the glacial period. This

change of land is quite as favorable to the collection of sand along the shore line as the process of elevation. In general the existence of great quantities of sand along an ocean shore may be taken as probable evidence of recent geological movements which have enabled the sea to work over a great amount of débris, of which it has left the heavier part where it found it, and has heaped up the lighter parts where its currents naturally swept them. It will hereafter be shown that the whole region of Narraganset Bay has less drift than exists in the regions further to the northward; it also has few stretches of shore which furnish quantities of sand to eroding agents and none of those great rolling beaches, such as abound to the northward where thousands of tons of pebbles, rushing to and fro under the beat of the waves, are gradually ground into sand and mud. The rocks exposed about Narraganset Bay to the full surge of the ocean are mostly stubborn resisters of the waves, and where masses break away they generally fall into deep water where they are not ground up by the waves. There are no currents working along the shore, which are capable of transporting sands from either the Cape Cod shore or the great masses of Long Island. We see that circumstances have cooperated to keep the inlets of this diversified and beautiful region of our shore free from the overwhelming sands, which in the regions a little to the east or west would have soon closed or effaced them. Nothing can be more strikingly contrasted than the topographical results of water in its solid and its fluid states. Where it moves from the land in the solid phalanx of the glacier it rends a coast into shreds, as ragged as a cloud blown out by the wind. In the sea with the ever varying action of waves and currents, it works to restore the uniformity it destroyed before. Its waves dig down the heights and fill up the hollows, its currents build moles across the inlets and give them over to the agents which speedily convert them to marshy plains.

At the close of each of the many great ice periods in the earth's history the fretted line of the fiord zone was probably swept clear of its débris of all kinds. Then began the process of occlusion which continued until the ice came again to renew its work. In recognizing the harbors and inlets of Narraganset Bay as glacial work, we get an example of the agent which has given nine-tenths of the havens of our seaboards. That the people of

the northern part of Europe have been universally maritime is

due, in no small degree, to the recurrent ice action of the northern hemisphere, so close is the connection between this most destructive agent and the highest life.

THE NEW IMMERSION ILLUMINATION.

BY R. H. WARD, M.D.

THE new illuminating lens, introduced by Mr. Wenham recently, has proved to be a sufficiently important accessory to the microscope to command more attention than it has yet received in this

Fig. 123.

country. A small plano-convex lens, nearly hemispherical, has the central part of its curvature stopped off with black varnish: and for convenience the part intended to be thus suppressed

Fig. 124.

may be ground away as shown in Figs. 123 and 124, the ground surface being subsequently painted black. The lens thus preparesi is temporarily attached by some highly refracting medium, such as

glycerine or oil of cloves, to the under surface of the slide. It becomes, manifestly, an immersion spotted lens; though it has lost so much of its angular aperture, to say nothing of the difficulty of placing the object in its focus, that it can no longer be used as such. Its available action is that of enabling us to throw light obliquely into the slide at such an angle, ordinarily impracticable, that it shall suffer total internal reflection from the top of the slide (Fig. 123), or from the top of the glass cover when that is optically identified with the slide, as when we examine an object in balsam, glycerine, etc. (Fig. 124).

For many years our best means of producing this effect was a prism, as shown in Fig. 125. A small prism is attached to the under surface of the slide, temporarily, by soft balsam or by oil or glycerine in the case of mounted specimens, or permanently,

by balsam to a blank slide which is to be used for the occasional examination of unmounted specimens. This arrangement gives so little light, and so little control of the angles at which the light meets the axis of the object and the axis of the instrument, that it has been but little used and with indifferent results.

Mr. Wenham's lens removes all these difficulties. It is easy to get light enough for moderately high powers; and the light can be thrown upon the object at a considerable range of angles and from any side or from all sides at once. When light is to be supplied from one side only, it may be directed by a rectangular prism or a Reade's prism, or an (inclined) achromatic condenser of long focus and small angle—such as a two-inch or three-inch objective; while from all directions at once it is best supplied by the common glass paraboloid. The latter effect may be obtained, as explained