June 15, 1888.] 



SCIENCE. 



281 



the expansive energy of the contained water that we owe, in part 

 at least, the upward movement of such materials. It is clear that 

 this is the case in true volcanic dikes, for all the phenomena of a 

 volcano indicate that the mainspring of its movements is to be 

 found in the vapor of water. The close likeness between ordinary 

 volcanic dikes and those which we cannot assuredly connect with 

 volcanoes leads us to the conclusion that all injections whatsoever 

 are most likely due to expanding vapors. Be this as it may, the 

 effects of dikes is to clearly remove the material from a great depth, 

 and place it in more superficial rocks. 



Although it is most likely that the crevices into which dikes find 

 their way may occasionally owe their dislocations to the action of 

 contraction attending on certain metamorphic changes, probably 

 the greater part of such ruptures are due to strains connected with 

 ■changes in the attitudes of the rocks. The dike material thus acts 

 as wedges to fill in all the cavities accessible to the igneous rocks, 

 as far as they are formed. It is evident, that, where this process 

 is numerously repeated, a considerable horizontal extension of the 

 rocks is necessarily brought about. Thus in many parts of New 

 England, as is well shown along its extended shore-line, where the 

 coast reveals the crystalline rocks, from one-tenth to one-twentieth 

 of the superficial area is occupied by such dikes. Generally, where 

 the conditions have been such as to induce an injection of dikes, 

 there is a large amount of vein matter deposited in the same field 

 which still further serves to produce an extension of area. Thus in 

 the region about Eastport the gain in the superficial area due to 

 these two causes amounts to somewhere near three per cent or 

 five per cent of the superficies exposed on the present surface of 

 the rock. 



Let us suppose that within any area of the earth's surface 

 the conditions are such as to favor, through the forces which 

 lead to vein-building and those which operate to create dikes, the 

 vertical migration of matter from considerable depths towards the 

 surface. The result on the tensions in the crust at such a point 

 will evidently be such as to favor the construction of mountains. 

 The constant abstraction of material from the depths will lead to a 

 diminution in the bulk of the deposits of that lower level, and a 

 • parallel augmentation of the strata nearer the surface. It may well 

 be that the differential contraction of the earth's mass, being greater 

 at lower levels than at higher altitudes in the section, may create a 

 slight tendency to buckle into mountain-ridges in all parts of the 

 ■crust : but, wherever this general contraction is combined with the 

 crenitic action, we may expect to find a more complete develop- 

 ment of mountain-chains ; and such points will be the seats of 

 folding, and they may by their wrinkles effect the necessary con- 

 traction of the crust, and thus prevent folding in other sections 

 where the conti-action of the whole sphere alone tends to produce 

 wrinkling. 



It seems to me that this hypothesis may, perhaps, explain the 

 fact that regions which have long been the seat of active sedimenta- 

 tion naturally become the sites of mountain-building. James Hall 

 and others have noted the fact, which so far has remained inexpli- 

 cable, that the first stage in mountain-building consists in the pro- 

 duction of extended sedimentary deposits of more than normal 

 thickness. During the deposition of these sediments the earth's 

 crust appears to be down-borne by their weight. After the subsi- 

 dence some action sets up which leads finally to a certain elevation 

 ■of the area, and consequently to a development of erosive action. 

 As the deposits are worn away, the mountains rise higher and 

 "higher, as the folding becomes more and more irrtense. 



Although the generalization concerning the formation of moun- 

 tains which I have just stated has not been critically compared with 

 the many instances of mountain-structure, it seeins of sufficiently 

 common occurrence to demand an explanation, and it very likely 

 will prove true for all large mountain systems whatsoever. Is it not 

 possible that we may account for the development of mountains 

 through these series of changes in the following manner? viz., 

 where, as along a shore-line, sediments are thickly accumulated, 

 the first effect may well be the down-sinking of the region ; then, 

 as the thickness of the stratified section increases, and the blanket 

 retaining the internal heat becomes deeper, the internal heat will be 

 greatly increased in the lower portions of the section. This will 

 induce an upward migration of the imprisoned waters, and conse- 



quently, in time, a transfer of material to higher levels in the rocks. 

 The consequent expansion of these superjacent rocks will make 

 them tend to buckle. The superficial strata may not have received 

 any considerable infiltration or injection of the material, yet they 

 may be contorted by movements in the subjacent rocks which have 

 thus been increased in volume ; in other words, an intensification 

 of deposition, if the sediments attain a great depth, may in time lead 

 to a reversal of the down-sinking movement and the construction 

 of a mountain system in what was previously a basin of sedimenta- 

 tion. 



This explanation of mountain-folds will probably not at all ac- 

 count for the development of the basilar uplifts or tableland elevations 

 which are developed in connection with all or almost all important 

 chains. It may well be the fact that the expansion of the overlying 

 deposits through the upward deportation of matter is only one ele- 

 ment in determining the formation of mountains. It may in the 

 end turn out that mountains are the result of a tolerably compli- 

 cated series of causations, in which secular refrigeration of the 

 earth, the transfer of weight by the operations of erosion and dep- 

 osition, and the subterranean migrations of matter, all take a part. 

 It may indeed well be the fact that these internal movements of 

 material are due to more than one cause. I am, however, inclined 

 to believe that to this vertical movement of materials we owe in 

 many cases a share of the conditions which bring about the forma- 

 tion of mountainous dislocations. N. S. Shaler. 



SCIENTIFIC NEWS IN WASHINGTON. 

 A New Instrument for measuring the Direction and Velocity of Sub- 

 marine Currents. — Cabinets of Typical American Rocks, for Use 

 in Colleges and Universities. — Beantiful Specimens of New Jersey 

 Serpentine. — Ojibwa Pictographs in the West. — The Yellow- 

 Fever has disappeared from Florida. — Interesting Phenomenon at 

 Sea. 



A Direction-Current Meter. 



The increasing commercial importance of our rivers and harbors, 

 and the recent large annual appropriations for their improvement, 

 have given a fresh impetus to the study of physical hydrography 

 and hydraulics. It has come to be pretty generally recognized that 

 no plans for the permanent improvement of tidal harbors, and such 

 streams as the IVlississippi and its tributaries, can be perfected with- 

 out a thorough knowledge of the physical laws which underlie the 

 complex phenomena they present. The investigation of these laws 

 has stimulated observers and experimenters to the invention of 

 many new and improved devices for the precise measurement of 

 the various factors involved. One of the most interesting of these 

 devices is a direction-current meter, recently perfected by Mr. E. 

 S. Ritchie, the well-known maker of philosophical apparatus, of 

 Boston, and iVIr. E. E. Haskell of the United States Coast and 

 Geodetic Survey. The characteristic feature of this meter is that 

 it gives simultaneous measures of the direction and speed of a cur- 

 rent. The direction is determined by means of a compass in all 

 respects similar to Mr. Ritchie's trailing compass, which is mounted 

 in an elongated chamber, whose axis coincides with the axis of the 

 meter (see accompanying cut). A system of electro-magnets and 



circuits connects the compass with a dial, which may be placed in 

 any convenient position, in such a manner that the observer may 

 make the dial indicate the same azimuth as the compass-needle. 

 The speed of the current is measured by a conical propeller-wheel, 

 whose flukes are curved in conformity with the requirements of 

 theory for maximum rotary effect of moving water, and whose 

 mass is as small as practicable with its requisite stability. The 

 revolutions of the wheel are counted automatically by an electro- 



