Natural cements will show much less-60 to 100 lbs. per square inch, as before stated, being their average strength.

The testing of an unknown brand would require a much longer time. Tensile strength tests after twenty-eight days, or even longer, must be made, as there are faulty cements which quickly attain a high strength, then gain no more, or even fall off in strength, or disintegrate in time. Faija states that a cement should gain an increase of twentyfive per cent. in strength at the end of twenty-eight days over what it has at seven days. Not only this test should be applied, but samples of the set cement should, after seven days, be set out in the air. Some cements will, under these circumstances, fall to powder gradually, and become worthless. Of course, this comes from faulty manufacture, probably, in the case of artificial cements, from over-claying the re. quired mixture.

The details for testing the tensile strength of cement are fully elaborated in the report referred to. The common test for cement, by which a mason estimates its quality, are of little value. Mixing it with a "mortar" and putting it on a brick serves to compare the cement with the ordinary kind he uses as to speed of setting, color, etc., but gives no safe guide to an opinion of its real strength. In fact, many light and over-clayed cements yield a more easily worked mortar than the true Portlands.

The weight and color of cement is a popular means of testing it, but the indications are very fallacious. The weight of an artificial cement is due, first, to the extent and hardness of the burning, and second, to the fineness of the grinding, the finer, the lighter, and the lighter burned, the lighter. The color of Portland cement depends partly on the nature and partly on the proportions of the materials. Over-clayed cements are usually reddish or "foxy," and such hues may therefore indicate weakness. The proper color is from a light to a darkgray, with a rather greenish cast.

In this short sketch of the nature, sources, and preparation of cements, it may be well to again call attention to the many advantages Ohio possesses as a place of manufacture of the artificial cements. We have numerous limestones which cannot be exceeded in purity as sources of carbonate of lime. With these are the drift and bedded clays of every variety, and furnace slags, slates and marls of all grades. There are many points in the state where cements of excellent grade could be cheaply and easily manufactured. A small works at Columbus is at present trying the experiment of working clay, limestone and slag. While connected with these works as chemist, it became the writer's duty to experiment on the different materials available; and while, from commercial reasons, the supply of material is now exclusively Columbus limestone and the plastic clays near Logan, in Hocking county, during the investigation excellent Portland cement was made from the drift clays of various points, with and without slag admixture. There are better limestones to be had in the state than the Corniferous used in Columbus, which has a considerable amount of gritty silica. At South Bend, Indiana, a fine Portland is now made from a drift clay and a bed of shell marl. The two are so soft as to be easily incorporated, almost without grinding, and burned to a most excellent "clinker." The cement is largely used in paving, and other work requiring good Portland.

CHAPTER XIV.

GYPSUM OR LAND-PLASTER IN OHIO.

BY EDWARD ORTON..

DISTRIBUTION.

Land-plaster or gypsum (sulphate of lime) is at present worked in Ohio at but a single locality, viz., the station of Gypsum, Ottawa county, on the Lake Shore and Michigan Southern Railway, ten miles west of Sandusky, and by only a single firm, viz., Marsh and Company, of Sandusky. The mineral has been known here since the first occupation of the country, coming to view in the rocky floor of Sandusky Bay, immediately adjacent to which the quarries are located. Not more than twenty-five acres have been already worked out, and there is probably as much more territory that has been proved to contain gypsum in quantity to justify working, while a larger acreage to which no thorough tests have yet been applied may be reasonably expected to hold valuable deposits. Most of the proved territory is included in the two hundred acres that Marsh and Company own, but in past years quite a large amount has been taken from the farm adjoining this, upon the west. The surface, which is composed of the usual Drift clays, is but a few feet higher than the waters of the bay. In the quarries that have been thus far worked, the drift has been very shallow, being confined mainly to troughs or hollows in the limestone, the results of earlier erosion.

GEOLOGICAL HORIZON.

The gypsum of the locality above described is derived from the great series of limestones which is known in the Ohioscale as the Lower Helderberg or Waterlime formation. This series is much the largest single division of limestones in the state. Its maximum thickness under cover does not fall short of 700 feet. In outcrop, we do not find more than 100 feet in vertical sections which can be connected in a combined section. The largest measurements of this sort are found in Highland county, near Greenfield. In the northern part of the state, there are frequently found, rising from under the drift beds that cover and obscure all of the bedded rocks, sections of a few feet of the Waterlime series, but it has not been possible hitherto to combine these isolated sections in any way so as to show the entire thickness of strata represented by them. The breadth of country occupied by the formation, however, is good enough proof that it has a thickness as great at least as that claimed for it above.

Gypsum appears to be distributed at frequent intervals throughout this series in this particular portion of the state. In the deep well drilled in 1886 at Sandusky, a bed nine feet in thickness was reported by the driller at a depth of 272 feet, or about 150 feet below the base of the Corniferous limestone. It was also reported in smaller amount at many other points in the next 800 feet of rock passed by the drill. It is, however, possible that the gypsum which actually occurs in the drillings from a considerable number of points in the descent may have been, in part, at least, derived from the highest deposits by the action of the rope and the passage of the tools in drilling.

At Port Clinton, also, three miles to the westward, gypsum was found at various points from 90 to 190 feet below the surface. These deposits are deeply buried at the quarries now under discussion and must be correlated with some of the lower beds at Sandusky, already reported.

The former reference of the gypsum of the quarries to the Salina group must be abandoned. Twenty-five miles west of this locality, viz., at Genoa, the Waterlime is found resting directly upon the Guelph beds of the Niagara. It carries here its most characteristic fossil, viz., Leperditia alta. It is 400 feet or more above this horizon that the gypsum beds are found.

The assignment of these gypsum beds to the Lower Helderberg series is rendered much less revolutionary than it would have been thought to be a few years since, by the discovery that some of the important plaster quarries of central New York are to be referred to this same series. This discovery was made by Professor S. G. Williams, and is recorded by him in the American Journal of Science, September, 1885.

At various other points, gypsum is found in the outcrops of the formation, and notably in the vicinity of Sylvania, Lucas county, while in the deep wells recently drilled through northern and central Ohio, it is the exception to miss deposits of gypsum in the samples of drillings. The last case reported is three feet of pure fibrous gypsum from a depth of 150 feet at Upper Sandusky.

MODE OF OCCURRENCE AND ORIGIN.

The plaster beds occur in the following sections, viz.:

No. 2. Bowlder bed carrying gypsum in separate masses im

beded in shaly limestones.....

5 feet.

...3-5 feet.

No. 4. Lowest plaster bed, variable......
Mixed limestone and plaster, bottom of quarry. Water enters here in quantity.

The beds are not even and horizontal, but are found in waves or rolls, the summits of which rise five to eight feet above the general level. Sections like the one here given will yield 50,000 tons of plaster to the acre.

The bed marked No. 1 in the section, is a mixed deposit of shale and plaster that has hitherto been rejected, but which has recently been found fully available for grinding into a dark-colored land-plaster. It has been lost by erosion in much of the territory already worked, and is not commonly counted among the valuable resources of the quarries.

No. 2 is one of the most interesting divisions of the section. Scattered through the calcareous shales, balls of gypsum, concretionary in form and probable character, varying in diameter from six to twentyfour inches occur. For a long while it was thought that they were of inferior value, and they were ground into land-plaster, but recently it has been found that the purest product of the quarries can be derived from these same plaster balls. The gypsum yielded by them when they have been carefully freed from their shaly envelopes, proves to be of the whitest and purest sort such as is used as terra alba.

But the division marked No. 4 is the main reliance of the quarries. It is from this that the chief supply of plaster for calcination is derived. The gray rock by which it is separated from the overlying section, is an even-bedded limestone of the most characteristic Lower Helderberg type. It contains a considerable percentage of gypsum distributed through the rock, as is shown in the following analysis: (Lord.)