The rock is thus divided into three sections, separated from each other by surfaces of dislocation, where the rock is much wrenched and broken. These places of dislocation are called the Northern and Southern Quebrada, from the Spanish ‘Tierra Quebrada,’ or broken ground; and it is at these places that the inland caves of Gibraltar are almost exclusively found. Based on the observations of Dr. Falconer and himself, an excellent and most interesting account of these caves, and of the human remains and works of art which they contain, was given by Mr. Busk at the meeting of the Congress of Prehistoric Archæology at Norwich, and afterwards printed in the ‘Transactions’ of the Congress.[47] Long subsequently to the operation of the twisting force just referred to, the promontory underwent various changes of level. There are sea-terraces and layers of shell-breccia along its flanks, and numerous caves which, unlike the inland one, are the product of marine erosion. The Apes’ Hill, on the African side of the strait, Mr. Busk informs me has undergone similar disturbances.[48]
In the harbour of Gibraltar, on the morning of our departure, I resumed a series of observations on the colour of the sea. On my way out I had collected a number of specimens, with a view to subsequent examination. But the bottles were claret bottles, and I could by no means feel sure of their purity. At Gibraltar, therefore, I purchased fifteen white glass bottles, with ground glass stoppers, and at Cadiz, thanks to the friendly guidance of Mr. Cameron, I secured a dozen more. These seven-and-twenty bottles were filled with water, taken at different places between Oran and Spithead.
And here let me express my warmest acknowledgments to Captain Henderson, the commander of H.M.S. ‘Urgent,’ who aided me in my observations in every possible way. Indeed, my best thanks are due to all the officers for their unfailing courtesy and help. The captain placed at my disposal his own coxswain, an intelligent fellow named Thorogood, who skilfully attached a cord to each bottle, weighted it with lead, cast it into the sea, and, after three successive rinsings, filled it under my own eyes. The contact of jugs, buckets, or other vessels was thus avoided, and even the necessity of pouring the water out afterwards through the dirty London air.
The mode of examination applied to these bottles after my return to London is in some sense complementary to that of the microscope, and may I think materially aid enquiries conducted with that instrument. In microscopic examination attention is directed to a small portion of the liquid, the aim being to detect the individual suspended particles. In my case, a large portion of the liquid is illuminated by a powerfully condensed beam, its general condition being revealed through the light scattered by suspended particles. Care is taken to defend the eye from the access of all other light, and, thus defended, it becomes an organ of inconceivable delicacy. Were water of uniform density perfectly free from suspended matter, it would, in my opinion, scatter no light at all. The track of a luminous beam could not, I think, be seen in such water. But an amount of impurity so infinitesimal as to be scarcely expressible in numbers, and the individual particles of which are so small as wholly to elude the microscope, may, when examined by the method alluded to, produce not only sensible, but striking, effects upon the eye.
The results of the examination of nineteen bottles, filled at various places between Gibraltar and Spithead, are here tabulated:
| No. | Locality | Colour of Sea | Appearance in Electric Beam |
|---|---|---|---|
| 1 | Gibraltar Harbour | Green | Thick with fine particles |
| 2 | Two miles from Gibraltar | Clearer green | Thick with very fine particles |
| 3 | Off Cabreta Point | Bright green | Still thick, but less so |
| 4 | Off Cabreta Point | Black-indigo | Much less thick, very pure |
| 5 | Off Tarifa | Undecided | Thicker than No. 4 |
| 6 | Beyond Tarifa | Cobalt-blue | Much purer than No. 5 |
| 7 | Twelve miles from Cadiz | Yellow-green | Very thick |
| 8 | Cadiz Harbour | Yellow-green | Exceedingly thick |
| 9 | Fourteen miles from Cadiz | Yellow-green | Thick, but less so |
| 10 | Fourteen miles from Cadiz | Bright green | Much less thick |
| 11 | Between Capes St. Mary and Vincent | Deep indigo | Very little matter, very pure |
| 12 | Off the Burlings | Strong green | Thick with fine matter |
| 13 | Beyond the Burlings | Indigo | Very little matter, pure |
| 14 | Off Cape Finisterre | Undecided | Less pure |
| 15 | Bay of Biscay | Black-indigo | Very little matter, very pure |
| 16 | Bay of Biscay | Indigo | Very fine matter, Iridescent |
| 17 | Off Ushant | Dark green | A good deal of matter |
| 18 | Off St. Catherine’s | Yellow green | Exceedingly thick |
| 19 | Spithead | Green | Exceedingly thick |
Here, in the first instance, we have three specimens of water, described as green, a clearer green, and bright green, taken in Gibraltar Harbour, at a point two miles from the harbour, and off Cabreta Point. The home examination showed that the first was thick with suspended matter, the second less thick, and the third still less thick. Thus the green brightened as the suspended matter became less.
Previous to the fourth observation our excellent navigating lieutenant, Mr. Brown, steered along the coast, thus avoiding the adverse current which sets in through the Strait of Gibraltar from the Atlantic to the Mediterranean. He was at length forced to cross the boundary of the Atlantic current, which was defined with extraordinary sharpness. On the one side of it the water was a vivid green, on the other a deep blue. Standing at the bow of the ship, a bottle could be filled with blue water, while at the same moment a bottle cast from the stern could be filled with bright green water. Two bottles were secured, one on each side of this remarkable boundary. In the distance the Atlantic had the hue called ultramarine; but looked fairly down upon, it was of almost inky blackness—black qualified by a trace of indigo.
What change does the home examination here reveal? In passing to indigo, the water becomes suddenly augmented in purity, the suspended matter has become suddenly less. Off Tarifa, the deep indigo disappears, and the sea is undecided in colour. Accompanying this change, we have a rise in the quantity of suspended matter. Beyond Tarifa, we change to cobalt-blue, the suspended matter falling at the same time in quantity. This water is distinctly purer than the green. We approach Cadiz, and at twelve miles from the city get into yellow-green water; this the London examination shows to be thick with suspended matter. The same is true of Cadiz Harbour, and also of a point fourteen miles from Cadiz in the homeward direction. Here there is a sudden change from yellow-green to a bright emerald-green, and accompanying the change a sudden fall in the quantity of suspended matter. Between Cape St. Mary and Cape St. Vincent the water changes to the deepest indigo. In point of purity, this indigo water is shown by the home examination to transcend the emerald-green water.