these cracks is often found to have moved up or down, bringing strata of different ages or different rocks, or different portions of the same rock, opposite each other. These slips or displacements vary from a few inches or less to many thousands of feet, and are common in all areas where there has been much folding or other evidence of regional disturbance, volcanic or otherwise. This establishment of cracks when there has been such a slipping is known as faulting, and these faults must be studied most carefully in connection with ore deposits. They are, of course, just as abundant in eruptive as in stratified rocks. So-called dikes of eruptive material have often come up through them, having found in such fissures a convenient avenue of escape. Moreover, it is largely through the agency of crevices formed in some such way that the majority of the deposits of mineral matter, which we know as ore deposits, have had their origin, for the simple reason that were it not for these fissures or deep-reaching cracks the hot waters, or perhaps in some instances vapors, which have produced many valuable deposits of ore, could never have come up with their load of mineral matter from the source from which they obtained it. The original sources of the elementary substances, which, when subsequently deposited, usually combined with one or more other elements, form many of our metalliferous deposits, will be perhaps forever unknown; for we will probably never be able to know sufficient concerning the interior composition of the earth at such depths below the surface as those

Fault in Southwest Virginia (Lesley).

Inclined and Vertical Faults.

A group of Faults (Geikie).

from which many of them have certainly been derived. Later faults, long after the deposition of the mineral matter had ceased, have often cut these ore deposits in two, and in some cases they cause the miner a great deal of trouble.

Soil. Soil is usually the result of the gradual decay of the superficial portion of the solid or consolidated rocks beneath, and which are said to be in place; that is, through the physical and chemical agencies of rain, frost, or other atmospheric action the superficial portion of the rocks has gradually become rotten and has slowly dissolved into soil. A great portion of the soil of the world is of this description, and we observe here, simply, the first step in the grand work of erosion, since solid rock must first be disintegrated by chemical or physical forces, or by both, before it can be removed by the rainfalls which carry off this material to the rivers and so down to the sea.

In some cases, however, soil is alluvial; that is, it is composed of material (usually mud with some vegetable matter, and often including pebbles and sand) which has been brought from some locality more or less distant from that now occupied by it through the action of water. Many old river and lake bottoms furnish soil of this description. Somewhat similar in origin is the kind of soil which is produced by the recent raising from underneath the sea, of unconsolidated marine sediments. Thus the upper portions of many late Tertiary and Quaternary strata now covering land areas have become what we know as soil. For example, there is much soil of this description in the southern portion of the United States. But, generally speaking, most of the soil of this country, as of the rest of the world, is produced by the exceedingly slow rotting of the upper portion of the solid rocks, stratified or eruptive, immediately underneath. It is thus easy to see that the value of land for agricultural as well as for mining purposes must always depend upon the geological conditions which may be found to exist over any given area. For example, limestone and some eruptive rocks, when they are superficially converted into soil, often make excellent farming land, while sandstone and other classes of eruptives usually make poor land, etc. Erosion. Another very essential point- and especially essential in the study of mineral deposits to which it is desired to call

In glaciated regions we often find that large quantities of boulders, pebbles,

and even soil have been shifted by the ice sheet.

attention is that one cannot overestimate the importance of the process which is known geologically as erosion, and which has operated everywhere on land areas since the earth cooled sufficiently to permit of oceans. Even those who have given much study to geological phenomena often fail to appreciate properly the huge mass of material which has been removed by the various agencies of erosion. These agencies are rain and the chemical atmospheric action mentioned, rivulets and rivers, the ever-acting agency of the sea beating upon a coast, glacial or ice action, etc. Glacial action, however, has not been nearly so important a factor as the others. All stratified rocks, often of many thousands of feet in thickness, have certainly been formed from the ruin or disintegration of previously existing rocks composing land areas which have been mainly eaten away by the sea, or by the action of rain falling upon, and of the rivers traversing, these areas. Eren when throwing out of consideration the immense thickness of the older stratified rocks, which of course owe their origin to the erosion of ancient and unknown land areas, it is at present possible to prove that upon many portions of the earth exposed to our examination several miles in thickness, or rather in height, of solid rock have been removed from the surface of a given area, At any place whatever it is impossible to form a correct idea of the huge amount of material which has certainly been worn away, carried off, and deposited somewhere else. Erosion can be observed and studied to best advantage in the case of stratified rocks, but we know nothing of the amount removed from the older rocks, sedimentary or eruptive, upon which, as the floor of some ancient sea, these sediments were laid. So on it goes, and probably will for countless ages, in what appears to be an endless cycle, always a tearing down and rebuilding, a never-ceasing conflict between the forces which elevate continents, and the rainfalls which endeavor to reduce these to the common level of the sea. This is one of the reasons that we so often find nothing remaining of the outpouring of eruptive material, but only find the neck of an old crater or else the so-called dike, which occupies the vent or rift through which this rock came up when in a molten condition. The famous Palisades of the Hudson are an example of the latter. Practically all mountains, no matter how high, and other topographical scenery are due simply to the irregular action of erosion which has carved out valleys, often, though not always,

indicating softer rock, and left standing the hills or mountain. ridges, which frequently mark the position of harder or more unyielding rock. But from above the tops of these very hills or great mountains themselves there is good reason to believe, and in fact we frequently know, that thousands of feet of rocky material have been removed by exactly the sanie agencies. Whole series of strata of great thickness have often been slowly bent and fearfully crumpled, and thus elevated over vast areas, and then afterwards greatly eroded, giving rise to effects that could not be easily understood were it not for the appreciation of the grand results accomplished by the denuding agencies mentioned above. This is going on at the present time, but with inconceivable slowness, yet perhaps quite as rapidly as it has during different portions of the past many millions of years. Of course the denuding and rebuilding agencies have operated somewhat more rapidly at certain times than at others, as in times when there was a great amount of precipitation, but, so far as is known, always with exceeding slowness. And yet we know that twenty or thirty thousands of feet of sediment, or even more, covering hundreds of thousands of square miles, were accumulated during a single geological age!

It is the appreciation of these things which gives one a slight inkling of the absolutely inconceivable vastness of geological time. It also makes any attempt to compute this time most difficult; for, so far as we now know, we have no data of a definite character to reason upon. In the author's opinion, any calculations based upon thickness of strata and other such data must of necessity be very unsatisfactory, because we know nothing whatever of the conditions amount of precipitation, etc. - which prevailed at the time these strata were laid down as sediments. we know is that "time is long," geologically considered, long, indeed, that the human mind cannot compass its vastness. The history of the world as recorded in its stratified rocks contains irrefutable evidence of an age equal to many millions of years, beyond which, however, lies that which Professor Le Conte aptly calls the "infinite abyss of the unrecorded."1

1 Lord Kelvin has recently given it as his opinion that the earth solidified between twenty and thirty millions of years ago. In the present stage of our knowledge, how

All so

ever, it is submitted that these figures may be very far from the truth; such a distinguished authority to the contrary notwithstanding.

Sections between River Valleys showing Erosion (Phillips).

Ideal Section north and south from Canada to Pennsylvania (Le Conte).

Anticlinal Fold.

Synclinal Fold.

In the diagrams it will be observed that where the strata have been subjected to enormous pressure applied horizontally, and producing the folding already described, valleys have often been carved out of anticlinal rather than synclinal folds. This is due simply to the fact that wherever a start has been made. in the top of sneh a fold, sometimes through the original and constant assistance of some crack, which is very often a fault, - rain or snow water running either way down the lines of stratification or of sedimentation has been able, though with infinite slowness, to carve out a great valley

1 Anticlinal folds are those where the strata present to the eye a convex ap

frequently a number of

pearance, whereas in synclinal folds they present a concave appearance.