510 



THE POPULAR SCIENCE MONTHLY. 



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, ulti- 

 mately 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 fig- 

 ures and a number of provisional assump- 

 tions, in order to reach conclusions of gen- 

 eral interest. These conclusions are, that it 

 would take, in round numbers, 20,000,000 

 years to accumulate the quantity of sul- 

 phates 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 sea- 

 water for the supply of the hard parts of 

 shell-fish, crustaceans, corals, and other ma- 

 rine 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. 

 Pealc, in the American Journal of Science, 

 thnt 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 Palaeozoic 

 time. In very early time in Coloi'ado there 

 was archsean land rising above the Palseo- 

 zoic sea. As the Carboniferous age pro- 

 gressed, this land diminished by encroach- 

 ment 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 Lig- 

 nitic there was a physical break, followed by 

 a subsidence (at least locally) and subse- 

 quently 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 con- 

 tinuing through the period, accelerated per- 

 haps at the close of the Lignitic, and the 

 deposition of at least Lower Miocene strata. 

 This elevation is probably still going on. 



The Bnilding 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 arti- 

 sans, 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 scarce- 

 ly dates before the year 1862, building has 

 made rapid progress. In 1867 it began to be 

 specially active, and since that time an aver- 

 age 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. 



