When in search of a rock in particular, the farm owners may be able to lead you to the very place. They may not have the same name for it that the geologist uses, but they are usually observing and are well acquainted with their immediate vicinity and the peculiar rocks around there, and they may save you time. They are generally acquainted with the "fossil-rocks" and coal beds, and may be able to point them out to you where you least expected to find them. Another point in this connection that bears watching is this, be sure that the formation you find is in place, watch out for land slips that may have dislocated the rocks from their original position. Exposures found along roads are to be carefully watched, and wherever possible search should be made in the fields, up gullies and ravines. Road and railroad cuts offer many good exposures, creeks should be followed, but exposures found in them are to be in place, and not merely found lying in the creek bottom. The_ geologist must do some clear thinking which in connection with his instrumental observations will bring good results.
Be on guard against false dips. Quite often a stratum deposited by strong currents is made up of inclined layers (cross beds) having the appearance of bedding planes which may be taken for dip. They are most often found in sandstones. (Fig. 51.) Contemporaneous erosion may also mislead one in this same way. This is caused by a current of water forming a channel during sedimentation, and when such a flow has stopped, the excavated channel may be filled up resulting in an irregular. line having a synclinal appearance. (Fig. 52.) The methods of obtaining dips in a territory where the dips may easily be seen with the naked eye is done by means of a clinometer and magnetic needle. Such instruments may be combined in one, such as the Brunton pocket transit. It is best to make clinometer measurement at points where the outcrop may be seen for a considerable distance, and then the true dip sighted with the clinometer in hand, at the upper or lower portion of the outcrop and sighted to the furthest visible portion of it.
Never pass up any wells that have been drilled in a territory in which you are working; and by all means try to get all the information possible about them. Locate them on the map and get the elevation of the ground, as well as the elevation of the top of the casing if there is one sticking out of the hole; as the measurements of the depths of wells are done from the derrick floor, which is generally on level with the top portion of the casing.
Seepages. In the course of field work one should always be on the lookout for seepages of oil and gas. In a country not previously prospected, and where the geological horizons are not well known, seepages are of great value in determining the possible oil- and gas-bearing horizons. Seepages may be either
at the outcrop of the strata or may be at a fault, and thus in each case the condition under which a seepage is found should be determined. Seepage may be from shale, and is therefore important only in determining the possible source from which oil may migrate into a suitable reservoir. A seepage at the out crop might indicate that the stratum is petroliferous, and in following the stratum down the dip away from the point of seepage and under good structure it may become a good prospect.
If the outcrop is well sealed by the residue of evaporated oil, the latter may be found close to the outcrop. A change in the Ethological character of a rock may also permit accumulations a short distance away. There are a great many pools so located, near the outcrop, and although generally small, yet are commer cially valuable, as they are shallow and therefore not expensive to operate. Outcropping sands may be analyzed to determine the presence of oil, by treating a crushed sample with ether; the hydrocarbons, if present, will be in solution, which upon evaporation will show an oil ring as a residue. Dark and black color will indicate an oil containing asphalt for a base and a light color indicates paraffin.
Do not be misled by floating of iron stain upon water. To the casual observer it is similar to a film of oil, but it is only iridescent films of iron hydroxide. The two may be distinguished simply by disturbing the film, and if it is iron stain it will break into irregular pieces, but in case of oil it will make round patches and will easily "run-together" again. Many useless oil excite ments have been caused by the finding of such an "oil" seep.
Depth and Thickness of Strata. In a country where the dips are moderate, the outcrop of a thick stratum cutting a hori zontal plane will give a rather exaggerated idea of the thickness of that formation. (Fig. 53.) In order that the true thickness may be known, it is to be calculated by means of trigonometrical formula;. This may be done by considering the distance of the outcrop along the horizontal plane as the hypothenuse of a right-angled triangle, the angle of dip being known, it is a simple matter to figure the thickness of that stratum. Similarly, when the distance below the ground a stratum may be expected, it is to be figured by trigonometry, the dip being known as well as the distance along the surface from the outcrop. In order that the unevenness of the surface may be accounted for the elevation of the outcrop as well as the elevation of the point where the depth of the strata is wanted should be known. A simple method of using alignment diagrams is given by Mr. Harold S. Palmer, in the U. S. G. S. Professional Paper No.