rain from every English square mile of the solid surface of the earth in the course of a year. All this dissolved matter, however far it may be transported by rivers, ultimately runs down into the sea. If, then, as commonly supposed, the sea contains only what has been washed out of the land, the results previously attained may help us to form some crude idea of the length of time which has been needed to give the ocean its present composition. Not to be irksome, we may pass over an array of figures and a number of provisional assumptions, in order to reach conclusions of general interest. These conclusions are, that it would take, in round numbers, 20,000,000 years to accumulate the quantity of sulphates of lime and magnesia contained in the vast bulk of the ocean, but only 480,000 years to renew the carbonates of lime and magnesia; with reference, however, to the latter constituents, it must be borne in mind that a vast quantity of carbonate of lime is constantly being removed from seawater for the supply of the hard parts of shell-fish, crustaceans, corals, and other marine animals, and consequently the amount calculated as present in the ocean is far from indicating the total quantity which is poured into it. But what are we to say of the chlorides, especially the chloride of sodium, which is the prime constituent of sea-water? The ocean contains so much of this salt, and the rivers usually so little, that we are driven to conclude from the author's calculations that it would take 200,000,000 years to renew the chlorides in the ocean!
Geological Changes in Colorado.—It is noted as an interesting fact by Dr. A. C. Peale, in the American Journal of Science, that Colorado, which now possesses the highest mass of mountains in the United States, and whose mean elevation is higher than that of any other State or Territory, was also one of the highest areas of the North American Continent in Palæozoic time. In very early time in Colorado there was archæan land rising above the Palæozoic sea. As the Carboniferous age progressed, this land diminished by encroachment of the sea, due to subsidence of the land. This subsidence continued through Triassic, Jurassic, and Cretaceous time, into the early Tertiary. At the close of the Lignitic there was a physical break, followed by a subsidence (at least locally) and subsequently by elevation, after the deposition of the Miocene strata. The elevation of the Rocky Mountains, as we now see them in Colorado, is the result of an elevation commencing in early Tertiary time and continuing through the period, accelerated perhaps at the close of the Lignitic, and the deposition of at least Lower Miocene strata. This elevation is probably still going on.
The Building System of Philadelphia.—A paper on "A Building System for Great Cities," published in the Penn Monthly, contains an account of what may be called the Philadelphia system, of separate houses for families of very moderate means—artisans, and even laborers. There are three primary forms of houses, viz., the two-story four-roomed house, the two-story six-roomed house, and the three-story eight-roomed house, the value of which is respectively $1,200 to $2,500, $2,500 to $3,800, and $3,000 to $5,000. They are always of brick, erected on stone-walled cellars not less than seven feet deep, fourteen by twenty-eight feet for the smallest houses, fourteen to sixteen by forty-two to forty-five feet for the six-roomed and the eight-roomed houses. All these are built in contiguous rows or blocks with a common wall between them. Since the inauguration of the system, which scarcely dates before the year 1862, building has made rapid progress. In 1867 it began to be specially active, and since that time an average of 4,500 houses yearly has been erected, of which 2,500 were two-story and 2,000 three-story. The writer of the paper in the Penn Monthly has learned, from a personal inspection of a district near the southern border of the city of Philadelphia, that three-fourths of the dwellings that have been erected two years or more are owned by those who live in them. In a space less than a mile square, and containing 4,000 dwellings, the proportion of vacant houses was less than two per cent. Certainly more than one-half of these dwellings were owned by their occupants, and often entire blocks of thirty or forty houses would show over ninety per cent, so owned.
