Isabela is the largest of the Galapagos Islands and is one that is not normally visited on boat cruises. Being still over the hot-spot of the earth's mantle, it is one of the younger islands and has 5 volcanoes, some of which are still active. It seemed that a trip to this island would be a good complement to our cruise, during which we had visited older and smaller islands in the archipelago. Hence, after a misty morning in Puerto Ayora, we lined up on the dock to board a 16 person "ferry" for the 2½ hour ride to Puerto Villamil on Isabela.

The ride was interesting, as the small boat raced up the big swells and almost surfed down their other side; we had a feeling of extreme vulnerability in such a small vessel way out in a very large ocean. Still, by the time Santa Cruz had disappeared from view, we were passing the broken cones of Los 4 Hermanos and the shoreline of Isabela started to appear. Not long after, we were surfing the waves into the calm harbour of Puerto Villamil, protected by flat jagged islets, remnants of long past lava flows.

The following day we were up early and sitting in the back of a truck with our new travelling companions heading off into the interior of Isabela; first across the barren black lava flats, which gradually became covered in tree cacti and palo santo trees, before climbing up into the cloud layer and denser, lusher garua forest, kept green by frequent mists. We were on our way up to climb Volcan Sierra Negra, 1400m high and, with a diameter of 10km, the world's largest crater of an active volcano, having last erupted in 2005. Soon the vegetation began to change again, becoming more shrubby as we emerged above the clouds to reach our destination at a grassy clearing in a local ranch, where our horses were waiting.

The first part of this trip was to be by horseback - a novel experience for the fair Nello and myself, both veterans of one ride. My previous ride was only 6 weeks earlier in Puerto Lopez and I had quite enjoyed it, riding with two others through the steep muddy trails of the San Sebastian cloud forest. This was to be a very different experience.

There were 16 in the group of riders and we set off climbing slowly up the dirt road. Pleasant at first as our horses strolled along in a drawn out file, we soon discovered that our arriero considered his role was to herd us all up the mountain as quickly as he could; whacking the trailing horses, sending them and their riders cannoning into those in front and so on, thus maintaining a tight jostling bunch of mainly inexperienced riders at fast trot up the narrow and uneven path which led to and then along the volcano rim.

It was difficult to admire the spectacular views and, given the amount of dust that our tight cluster of horses generated, it probably did not matter much that we were concentrating more on horses than the surrounds. With all the equine surging, I suspect that I had my knee up a horse's khyber more times in one hour than in the rest of my life. Nello was spared this indignity as her horse was a natural leader.

We were pleased to finally get off, high on the rim of Volcan Sierra Negra, to at last admire the size of the crater and the broad river of black lava from the 2005 eruption, flowing down from the eruption zone on one flank and running alongside the steep inner wall. To the north we could see the broad flat cones of Volcans Wolf, Darwin and Alcedo, looking like inverted plates beyond the narrow isthmus of central Isabela, the blue of the sea on both sides and the band of cloud below us to the east - an amazing panorama.

The walk back was quick, if a bit hot, with the heat bouncing off the dark lava. The only downside was that, after lunch in the shade of a large and gnarly tree, we had to mount our horses once again to be herded back down the volcano in a jostling cloud of dust. We finally arrived, covered in a thick layer of brown grime. I only have two comments to make - first, dust and contact lenses do not mix at all and, secondly, if this is the pleasure of horse-riding, pass me my trekking boots.

An hour later we were back at Puerto Villamil and heading out into the harbour to check out Los Tintoreras, the jagged low islets that protect the harbour here. They are home to marine iguanas, blue-footed boobies, sea-lions and white-tipped reef sharks, which during the day rest in large numbers in long channels in the fractured lava.

The latter, being something new, were the most interesting for us, though we were disappointed that they did not join us when we went snorkeling in the bay nearby. However, a pair of spotted eagle rays were more obliging.

After an idle morning, we caught a water taxi across the harbour for a short walk to stretch the muscles that were complaining about yesterday's horse-ride. We followed the Sendero de las Grietas, which took us past a small, coral sand cove, past a small salt flat, across an ancient lava flow that had fractured into tesselated polygons, to Las Grietas.

Here lay a 20m deep crevice in the lava, at the bottom of which fresh water infiltrating from the mountains and salt water infiltrating from the sea had created a long, deep pool of crystal clear sapphire coloured water; a beautiful spot where you can sit and reflect on the world about you in the silence of a sanctuary.

On the way back, we stopped at Finches Bay to sit in the shade and watch the sandpipers and lava herons go about their business. It seemed a fitting end that a small flock of Darwin's finches arrived to sit on our feet and hands, twittering and cheeping. Maybe they were just checking us out from breadcrumbs, but we took it as an appropriate farewell from the inhabitants of the Galapagos.