about equal portions of quartz and felspar are contained in it, both copper and tin are found, but neither of them in large quantities. The characteristics of clay-slate peculiarly suited to the production of lead ores are, -alumine in large proportion; colour, light blue, and then decomposing, or else not so blue, and then less decomposing. Clay-slate producing lead contains a large portion of alumine, and is not crystalline. Zinc ores (black jack) occupy a position between copper and lead, not unfrequently mixed with both. Other varieties of clay-slate, nearly, if not wholly, barren of metallic ores, occupy large tracts of the surface of the two counties. They are found passing, by almost insensible transitions, from the flooring and roofing slate, through the dry shelving slates, forming many parts of our cliffs and comparatively barren lands, down to deposits of muddy matter. Rocks of every kind, containing a large proportion of felspar, indicate the presence of yellow copper ore, provided the lodes traversing them are of a fair size and suitable direction. Rocks of that character increase in productiveness as the fracture becomes rough and uneven. It is especially the presence of felspar, in large proportions, that characterizes the rocks throughout Cornwall and Devon, where large deposits of copper ore have been found. These are deep blue compact clay-slate, greenstone, elvan, and the whitish clay-slate, and in a very few localities granite. Granite characterized as quartzose indicates the presence of black and grey copper ores. Lodes in the soft kind of granite, containing large quantities of quartz, commonly yield grey copper ore in great abundance. Copper lodes, receding from the felspar rocks, and approaching to primitive granite, are almost always found to yield tin. If such copper lodes, however, are found in mines, at a distance from such granite, to fail in depth, they do not contain much tin. This fact is in accordance with a principle already adverted to, that tin lies nearer to primitive granite than copper. Before closing these remarks I beg to notice a very common saying, employed by individuals too indolent to observe for them selves. They assert that there is no knowing where the ore is, or, as they more usually phrase it," where it is, there it is." I readily acknowledge, that, up to the present time, great uncertainty exists as to the result of mining operations. We are not only ignorant of the origin and causes of metalliferous deposits, but, in too many instances, of the phenomena that accompany them. From this lack of knowledge the most prudent miners are often at fault; and though enabled to say, with almost unerring certainty, under what circumstances profitable mines cannot be found, they are not equally able to foretell where they can be. It is something, however, to be enabled to prevent the misapplication of capital, in exploring barren rocks, and localities where experience has satisfactorily proved that metallic ores are not to be found in sufficient quantities to pay the cost of extracting. I feel confident that continued and careful observation, of all the circumstances developed in connexion with our rich deposits of metallic ores, would effect much more than this. We shall find order, system, and arrangement here, as well as in every other department of Nature. CHAS. THOMAS. Great Circle and Composite Routes, to and from Australia, &c. SINCE 1848, when Mr. J. T. Towson favoured the members of the Royal Cornwall Polytechnic Society with his first public Lecture on Great Circle Sailing, and explained the nature of the tables which he had constructed for facilitating the practice of this important branch of navigation, the great practical value of his invention has been very widely acknowledged. The utility of the sailings which he named composite and windward great circle, has also since the period alluded to been demonstrated by a number of unprecedented voyages to and from our antipodes. Through the kindness of this gentleman the members of the Polytechnic Society are now presented with a Chart, shewing some great circle and composite routes which have been found to singularly shorten the Australian voyage. The composite track takes advantage of the great circle route as far as convenient or practicable; but is modified by geographical or meteorological circumstances, and combines the knowledge of the geometer with that of the physicist and practical seaman. Thus, while Mr. Towson's Tables enable the navigator to adopt with ease tracts that before were surrounded with difficulty, they allow full scope to the skill and ability of the navigator. The records of recent voyages to Melbourne afford ample illustration of this fact, and make a most interesting chapter in the history of modern navigation. Notices of some of the more remarkable voyages may be found in the Transactions of the Philosophical Society of Liverpool for the last year, and the following communication from Mr. Towson gives the latest information respecting some of the routes indicated in the chart. It will be remembered that Mr. Towson presented his Tables to the British Admiralty for the use of the country, the only condition being that they should be published immediately, and at a low charge. The merchants of Liverpool have shown their appreciation of the utility of these Tables by appointing their inventor to the office of Examiner to the Local Marine Board, and, in acknowledgment of his services in connexion with navigation, have since very liberally subscribed towards a fund to provide for the future. The Each year's experience affords further information, in connexion with the homeward passages from Australia on the composite route by the Horn. We have at present come to the conclusion that the maximum latitude of 60° or 61° South latitude should only be adopted in the Austral summer months; at other periods the maximum latitude of 57° or 57° 30' should be preferred. Several recent voyages have confirmed the value of the recommendation, that the Horn and Falkland Islands should be sighted. homeward passages of the "Golden Era" and the "Red Jacket" afford very striking instances of the impropriety of keeping on the parallel of the maximum latitude, on or eastward of the meridian of the Horn. Ice is more frequently met with as far north as 55° South latitude, on the meridian of 45° West, than it is in 60° South latitude, to the west of the meridian of 70° West. In the cases above referred to, the "Golden Era" was in the greatest danger, by keeping on a high South latitude, when east of the meridian of the Horn, and the voyage was prolonged to the period of 150 days; while those of the "Marco Polo," the "Lightning," and the "James Baines" have ranged between 63 and 77 days. In each voyage these ships sighted the Horn and Falkland Islands, following one of the tracks described in the chart. When these tracks were first recommended it was on account of the winds and currents which were most favourable for insuring a rapid passage; but it has now been discovered that no route from Australia affords so great an immunity from the dangers of ice. By sighting the Horn and the Falkland Islands we leave to the south and east the only region bordering on the tracks to and from Australia in which considerable danger arises from this cause. Within this part of the ocean, ice has been met with during each season of the year, and in such quantities as greatly to embarrass the mariner when imminent danger has not been incurred. On the accompanying Chart is shown the ice which is recorded to have been seen, both near the tracks out and home, in the years 1854-5, the season of the year being distinguished by different colours: green represents the ice seen during the Austral spring months, and red, yellow, and blue, summer, autumn, and winter ice, respectively. F extraordinary passage out in 64 days on this parallel. also satisfactory to record that the "James Baines" made her with it is greatly increased by adopting a higher latitude. It is free from ice as the lower parallels, but the probability of meeting The outward passage by the latitude from 50° to 51° appears as afforded the foregoing data, and also the latitudes and longitudes The following list gives the names of ships whose logs have in which ice was observed: Utopia April 25 to 26, 1854 54° S. .... 80° E. ...... 57° to 55° S. 60° to 51° W. Pack ice. |