88 



NA TURE 



[Nov. 27, 1884 



is taken of the galvanometer, first to left, then to right, 

 and the shunt-resistance is then adjusted until the scale 

 reading is 53J millimetres on either side of zero, making 

 a total of 107 millimetres. We then know that a deflection 

 of 1 millimetre right or left will be produced by an electro- 

 motive force of 1/200 of a volt. The cell whose electromotive 

 force is to be tested is then substituted at B in place of 

 the standard cell, and readings taken right and left ; these 

 are added, and divided by 100, giving the E.M.F. of the 

 cell directly in volts. 



To measure currents the same calibration is made with 

 a standard cell. In the circuit of the current to be mea- 

 sured is interposed a wire of some small but accurately- 

 known resistance — for example, a standard I ohm, or, for 

 stronger currents, a standard wire of 01 ohm. The 

 two extremities of this coil are then connected to the 

 key (Fig. 3), the 10,000-ohm coil being interposed as 

 before. If the current to be measured is I ampere, it 

 will, in passing through the standard 1 ohm, produc 



Fig.3. 



between its ends a difference of potential of 1 volt, and 

 this difference of potential will, when readings are taken 

 right and left, give a total deflection of 100 millimetres to 

 correspond to I ampere of current. It is not difficult to 

 modify the arrangements so that the galvanometer may 

 measure, on the one hand, millionths of an ampere, and 

 thousands of amperes on the other. We have found the 

 instrument specially valuable for indicating rapid fluctua- 

 tions of current in experiments on the induction of currents 

 in armature coils when moved in a magnetic field. Its 

 complete aperiodicity and the very small inertia, both 

 mechanical and electrical, of its coil, render it most valu- 

 able for such work. The only defect— and that not a 

 serious one — observed in three months of use, is a slight 

 sub-permanent torsion on the suspending wires after 

 taking a large deflection ; but the method of taking double 

 readings, first to right, then to left, eliminates any error 

 that might arise from this cause. 



THE BASALT-FIELDS OF NEW MEXICO 



GEOLOGISTS interested in the history of the younger 

 lava-floods, by which such vast areas both in the 

 Old World and m the New have been deluged, will be 

 glad to know that Capt. Dutton, of the United States 

 Geological Survey, after a careful study of the modern 

 volcanic phenomena of the Sandwich Islands, has under- 

 aken the investigation of the basalt-territory lying in 



New Mexico, to the east and south of the area already so 

 fully described by him in his Monographs on the Utah 

 plateaux and the Canon country. It was originally 

 intended that he should have charge of the Survey of the 

 Cascade Range. This arrangement was changed at the 

 beginning of this last season. The Cascade ground was 

 intrusted to his able assistant, Mr. Diller, while Capt. 

 Dutton himself struck southward for a region in New 

 Mexico, which he had long wished to study, from the 

 light which he believed it would throw upon some of the 

 later phases of volcanism in the Western Territories. I 

 have received a long letter from him, written in his camp 

 at the San Mateo Mountains, from which, with his per- 

 mission, I send the following extract for publication in 

 Nature. Arch. Geikie 



Our wonderful Plateau Country we have known only 

 in part, and the portion we have studied most is situated 

 upon the western and northern side of the Colorado. 

 Numerous geologists have hurried over the southern and 

 south-eastern portion ; but so rapidly have they been 

 obliged to move in order to keep pace with the expe- 

 ditions of which they were mere appendages that very 

 little systematic knowledge could be gained. During the 

 last two years our topographers have made some excel- 

 lent maps of this region, and everything is ripe for the 

 geologist. 



I have described the western and southern portions of 

 the Plateau Country as being very sharply defined in a 

 geological as well as in a topographic sense. I think it 

 will in great part prove to be equally well-defined in the 

 south-eastern portions. Already it is clear to me that 

 the Rio Grande River constitutes a portion of that 

 boundary in this territory. Everywhere within range of 

 my present field the strata characteristic of the Plateau 

 Country rise gently from the Rio Grande to the westward. 

 Cliffs, mesas, and terraces, carved buttes, and gorgeous 

 colours are as typical of this region as they are of Utah 

 and Northern Arizona. There is, however, more of the 

 Cretaceous system preserved, and rocks of that age pre- 

 dominate, though the Trias and Permian are magnificently 

 exposed. Indeed, the Vermilion Cliffs of Southern Utah 

 have reappeared here in all their grandeur and glory, with 

 but slight changes of detail. 



But the features which are engaging my particular 

 attention at this moment are the volcanic vestiges. This 

 region has long been known under the mysterious name 

 of Malpais — mysterious, however, only to those who have 

 not read Humboldt's account of the malpais of Old 

 Mexico. All the mesas, or platforms of sedimentary 

 beds, within three or four miles of my camp, are sheeted 

 over with basalt. The lava caps are not ordinarily more 

 than fifty or a hundred feet thick ; though just around 

 me, in the very centre or focus of all, it becomes much 

 thicker. In the valley-plains, also, are found many sheets 

 of lava. But while the lavas upon the higher platforms 

 and terraces are ancient, those in the valleys are very 

 young. The centre of the activity has been (so far as 

 concerns my present vicinity) the San Mateo Mountains. 

 This name is synonymous with Mount Taylor, for the 

 "Mountains" consist of a single volcanic pile (11,380 

 feet) carved into numerous spurs by magnificent gorges. 

 It is a small Etna, built originally by outbreaks from its 

 flanks as well as its summit. But the spread of the lavas 

 from this centre is remarkable. To the north-north-east 

 they reach out in unbroken continuity for forty-five miles, 

 and for eighteen to thirty miles in the other directions. 

 The lava beyond the immediate base of the mountain- 

 cone is not thick. It forms a superficial sheet only on 

 each mesa, or table, with a thickness varying from 50 to 

 200 feet. 



The lava-capped strata have been cut into isolated 

 mesas by subsequent erosion, and gaps of two or three 

 miles sometimes separate one of these outliers from its 

 parent mass. 



