Narrative of the surveying voyages of his majesty's ships Adventure and Beagle (vol.1): between the years 1826 and 1836 : describing their examination of the southern shores of South America, and the Beagles's circumnavigation of the globe

Feb. 1827. GEOLOGICAL NOTICES. 43 Having accomplished our object, we began the descent. lu a comparatively mild and agreeable spot, I again set up the theodolite and barometer, while some of the party employed themselves in fruitless attempts to kindle a fire. The height, by the barometer, proved to be 1,845 feet above the sea ; and tlie bearings from this station were much better than those I had taken from the exposed summit. We reached our tent at noon, having been absent seven hours. At three we reached the beach, where the barometer stood at 29,312 (air 61. °3,* and mercury 62,°5). Excepting near the sea, where clay-slate (very similar to that of Point St. Anna, but with an opposite dip) showed itself, the side of the hill is clothed with trees and underwood, and no rock is visible until one arrives at the ravine. Around the summit of Mount Tarn the ground is bare, but so covered with small decomposed fragments, that the solid rock only appears occasionally : it is very hard, and breaks with a conchoidiil fracture : some of the specimens which we detached bore indis- tinct impressions of organic remains. We also found, project- ing from the rock in which they were embedded, nodules, or small rounded masses of stone, in an advanced state of decom- position, mouldering away in laminar forms somewhat resem- bling the inner leaves of a cabbage. Several were brought away carefully, but before we arrived on board they had crum- bled to pieces : the nucleus was quite hard, but was surrounded by concentric laminae, more brittle the nearer they approached to the outer surface. It seemed as if the face of the summit The above being the difference in the short space of three feet apart; the instrument, in the first case, being- just under the lee of the rocky summit of the mountain, and in the last, above it, exposed to the wind. * The air was so dry this afternoon that I failed to procure a deposit of dew upon Daniell's hygrometer, although the internal temperature was lowered from 61° to 37°. One of Jones's portable hygrometers was also tried, and the temperature was lowered to 31°^ without a deposit ; so that, the difference being more than thirty degrees, the expansive force of the air must have been less than 212, the dryness, on the thermometric scale, less than 367, and the weight of vapour, in a cubic foot of air, less tlian 2,355 grains.