the Bendimahi and Bitlis rivers, and thus brought Lake Van into being. The history of the Nimrud volcano may be summarized as follows from the speaker's observations : 1. Its forerunner was the Kerkur Dagh on its southern flank,a denuded mass of grey augite-trachyte, rising to 9000 feet, and crowned by many peaks. It was probably erupted in the Pliocene Period, subsequently to the folding of the Armenian area, in which the latest folded rocks are of Miocene (Helvetian-Tortonian) age, occurring north of the Nimrud Dagh and consisting of limestones with corals (Cladocora articulata, Orbicella defrancei, &c.), Lithothamnion, Foraminifera (Lepidocycline Orbitoides, Amphistegina, &c.), beds of Pecten (P. urmiensis, &c.) and of oysters (Alectryonia virleti). Nimrud and the other numerous volcanoes of Armenia came into existence at a period when the sedimentary rocks could no longer be folded, but were fractured along definite lines, and Nimrud is situated on the great fracture transverse to the Armenian folds at the apex of their bending round from the Antitauric (west-south-west to east-north-east) to the Persian (northwest to south-east) direction, and it also marks the point of intersection of this fracture with a great north-east to north-west fracture (Caucasian direction), which delimits on the south Lake Van and the faulted depression of the Plain of Mush, abruptly cutting off the Tauric horst of pre-Devonian marbles and micaschists. 2. Numerous flows of augite-rhyolite built up the vast cone of the Nimrud Dagh, and the increasing pressure on the central vent became relieved by extrusions of augite-trachyte along radial fissures, forming the present promontories of Kizvag, Zighag, and Karmuch. 3. A presumably long period of inactivity was followed by violent explosions destroying the summit of the cone, and from this crater (smaller than the present one) vast lava-flows of a very fluid basalt (crowded with phenocrysts of labradorite, pale-green augite, and some olivine) flooded the country and filled up the Bitlis and Akhlat valleys, which have since then been eroded a little below their former depth. The Sheikh Ora crater of basic tuff (now breached by Lake Van) probably belongs to this period. 4. Further explosions widened the crater in which a large lake was formed, while the eastern half of the crater became filled by a succession of outflows of augite-rhyolite, in which numerous blowholes were drilled, bringing to the surface large blocks of basaltic agglomerate and also affording sections showing the transition downwards from obsidian, spherulitic obsidian, and spherulitic rhyolite to banded augite-rhyolite (with sanidine and green augite in a micropacilitic ground-mass). 5. The last eruption was recorded in 1441 by a contemporary Armenian chronicler, and resulted in the extrusion of a very viscous augite-rhyolite along a north-to-south zone of weakness, both inside the Nimrud crater where it separated off part of the large lake to form the shallow, so-called 'hot lake,' and also to the north of Nimrud, where it rose up fissures and in a small crater. 6. A violent earthquake in 1881 which destroyed the village of Teghurt, at the eastern base of the crater-wall, was the last sign of activity; but earthquakes are still frequent in the Plain of Mush at the western foot of the Nimrud Dagh, and recent fault-scarps are clearly visible along the borders of this faulted depression. The speaker mentioned that he had presented his model of the crater to the Museum of Practical Geology (Jermyn St.) and the rocks and slides to the British Museum (Natural History), where his fossils from Armenia are already preserved. XV. Intelligence and Miscellaneous Articles. COUPLED CIRCUITS AND MECHANICAL ANALOGIES, To the Editors of the Philosophical Magazine. GENTLEMEN, CENTENARIAN PERIGAL is clean forgotten today, and his valuable kinematic work on his lathe. His method should be revived of drawing the ellipse or other Lissajous figures of combined vibration, as on p. 515, fig. 2. The enveloping rectangle is divided up into elementary rectangles by lines spaced, not equidistant, but in equal time of simple vibration. Perigal does this by describing a semicircle on each side of the rectangle, and then produces the ordinates of points at equal angular interval round the circumference. Starting at any point of crossing and tacking across the diagonal of an elementary rectangle, a succession of points is made on an ellipse inscribed in the rectangle, and the points are close enough to be joined up in a continuous curve, such as Perigal could cut in his lathe. If m and n steps are taken for a diagonal, the Lissajous curve appears for a combination of two vibrations of m and n fold frequency, and the phase difference of lead or lag is settled by the position of the starting point. Dec. 13, 1917. 1 Staple Inn, W.C. 1. Yours sincerely, |