"That is it," said the major, "no doubt about it. To keep off the ague. Do you not know that burnt clay is a sure protection against ague, which was the curse of the Essex marsh land? In Central Africa, in the districts that lie low and there is morass, the natives are quite aware of the fact, and systematically form a bed of burnt clay as a platform on which to erect their hovels. Now look here, my dear friend, I'd most uncommonly like to take a boat along with you, and explore both sides of the Blackwater to begin with, and its inlets, and to tick down on the ordnance map every red hill we can find." "I am quite ready," I replied. "There is one thing to remember. A vast number of these hills have been ploughed down, but you can certainly detect where they were by the colour of the soil." Accordingly, on the next fine day we engaged a boat--not a rower--for we could manage it between us, and started on our expedition. The country around the Blackwater is flat, and the land slides into the sea and river with so slight an incline, that a good extent of debatable ground exists, which may be reckoned as belonging to both. Vast marshes are found occasionally flooded, covered with the wild lavender, and in June flushed with the seathrift. They nourish a coarse grass, and a bastard samphire. These marshes are threaded, cobweb fashion, by myriads of lines of water and mud that intercommunicate. Woe to the man who either stumbles into, or in jumping falls into, one of these breaks in the surface of land. He sinks to his waist in mud. At certain times, when no high tides are expected, sheep are driven upon these marshes and thrive. They manage to leap the runnels, and the shepherd is aware when danger threatens, and they must be driven off. Nearer the mainland are dykes thrown up, none know when, to reclaim certain tracts of soil, and on the land side are invariably stagnant ditches, where mosquitoes breed in myriads. Further up grow oak trees, and in summer to these the mosquitoes betake themselves in swarms, and may be seen in the evening swaying in such dense clouds above the trees that these latter seem to be on fire and smoking. Major Donelly and I leisurely paddled about, running into creeks, leaving our boat, identifying our position on the map, and marking in the position of such red hills or their traces as we lighted on. Major Donelly and I pretty well explored the left bank up to a certain point, when he proposed that we should push across to the other. "I should advise doing thoroughly the upper reach of the Blackwater," said he, "and we shall then have completed one section." "All right," I responded, and we turned the boat's head to cross. Unhappily, we had not calculated that the estuary was full of mudbanks. Moreover, the tide was ebbing, and before very long we grounded. "Confound it!" said the major, "we are on a mudbank. What a fix we are in."