mulations of oil near the surface thus form a secondary deposit, which, however, differs essentially from the vein-like type previously discussed as being similar to a bed.

When the fissures K, Fig. 2, cross a bench or a group of strata of very porous rock CD, the oil may accumulate in it. Such secondary deposit shows the bed-like type of the rockbench, and frequently cannot be distinguished from an original

FIG. 3.

Q

deposit. Thus, for instance, the rich Pennsylvania oil deposits which belong to the sandstones and conglomerates imbedded concordant to the schist are by many supposed to be not primary, but secondary deposits.

Petroleum occurs also in small hollow spaces in the eruptive rocks. But this occurrence being of a very subordinate nature is only of scientific interest. Thus, small quantities of oil have been found in the diorite of Gaspé, Canada, in the melaphyres and similar eruptive rocks of Central Scotland, in the basalts at the foot of the Etna, and in Auvergne. The content of bitumen of many northern granite boulders which were found near Kiel (Dorfgarden, Pappenberg,) is well known. Small quantities of bitumen are carried here and there by granite in Cornwall (Poldice) and in Auvergne (Chamalier, Clermont); by the greenstone trachyte of Parad (Hungary), the melaphyre of Bohemia (Rybnik, Semil), and of the Palatinate of the Rhine (Oberstein).

The massive rock, during its eruption either traversed an oilbearing or bituminous rock, or crossed a coal measure, which, by reason of the high temperature and consequent destructive distillation, yielded petroleum-like products. Such occurrences must also be classed as secondary deposits. Deposits of petroleum may, therefore, be:I. Primary (impregnated beds and strata). I. In fissures.

Every secondary deposit presupposes the existence of a primary deposit; the yield of one, however, cannot be judged by that of the other. If the primary deposit was, for instance, located in slate-clay, which is capable of absorbing and maintaining large quantities of oil, and this slate-clay was later on crossed by fissures, the superfluous oil might pass into them. and accumulate, so as to form productive secondary deposits, while in the primary deposits but small quantities of oil are yielded by strongly bituminous schist.

There are many erroneous ideas regarding petroleum deposits. The expression "subterranean oil reservoirs" is ex

plained to mean that in the crust of the earth there are large caverns filled entirely or partially with crude oil, and even in literature is frequently found nearly the same sketch of such a lenticular cavern, which is said to be filled on the bottom with water, in the centre with petroleum, and on top with gases. Such caverns may exist, but their existence has never been established, and it is about time that this "hereditary woodcut" should disappear from literature.

Oil Lines.

It is a fact recurring in various regions that the productive oil wells are disposed in a certain direction, and that the productive territory lies in long narrow areas. Thus, for instance, the lower oil territory of Pennsylvania, south of Franklin, is 2 miles wide and 35 miles long. This peculiarity as regards the above-mentioned region was first observed, in 1867, by C. D. Angell. While investigating the occurrence of petroleum, he studied the relative positions of the most productive wells, and upon this basis erected the so-called "belt-theory," which rapidly reduced the risk in searching for oil from about 25 per cent. mis-drills to from 3 to 5 per cent.-certainly an extraordinary success.

A line laid in the centre of such a productive oil territory, stretching in the main in one direction, is called an oil-line; it indicates the way in which later bore-holes or shafts have to be disposed in order to secure the greatest probability of success.

In the United States the oil-lines were first drawn upon the basis of simple experience. In many other regions it might also be advisable to see whether the most productive wells do not correspond to a line, and in such case to arrange the next borings in the continuation of this line. In a region not yet opened the oil-line-in case the existence of one can be supposed-will have to be drawn upon a theoretical or hypothetical basis; the better founded the latter is, the greater the probability of success.

To connect two or several points where oil has been found

by a straight line, and declare that to be the oil-line, as has unfortunately been repeatedly done, is not only useless, but may divert attention from the proper way and bring a territory into discredit.

The extension in definite directions, which can be accurately shown in several oil regions, may be connected, Ist, with the extension of the oil-bearing stratum; 2d, with an anticlinal, and 3d, with a displacement.

These cases will here be discussed in detail.

1. Oil-line corresponding to the oil-bearing stratum.

a. The deposit belonging to a stratum stretches in the form of a plate in courses and dips.

If an oil-bearing group of strata E, Fig. 4, which crops

FIG. 4.

out on the surface at e, stands quite steep, the borings will rapidly assume great depths the further they are from e e. The operations will, therefore, be chiefly carried on in courses of the oil deposit E e, as indicated in the sketch by dots which mark the borings. If this is sketched in a map an oil-field stretching in length corresponding to the courses of the strata is obtained without an actual oil-line being in this case present, because this arrangement of the working has nothing to do with the distribution of oil, it depending only on the slighter difficulties and expense of labor and money.

This linear arrangement of the workings will be the more

pronounced the steeper the strata, and the deposit imbedded concordant with them, stand. If the latter assume a vertical position, Fig. 5, the oil may be found in a very narrow tract corresponding to the size of the deposit in the direction of the bearing of the stratum. If the latter is a straight line, the oil

line sketched in the map will also be a straight line, no matter how undulating the surface of the ground may be. Any bends in the bearing of the deposit, which have to be considered in sinking the bore-hole or shaft, will be recognized from the conformable bends of the strata accompanying it, so that in this case a thorough study of the stratum will be of great advantage in the working.

As regards the oil-line and the working, the conditions remain the same as explained above; in case with a vertical position of the deposit the strata incline towards right and left and become more level, thus forming an inverted fan. In this case a pronounced oil-line is also present.

b. The deposit corresponding to a stratum possesses a linear extension. Beds of sandstone and conglomerate need not always show the form of a plate, but with a comparatively small width and thickness may stretch to a considerable length in one direction. Thus, for instance along rivers, the beds of