According to T.C. Swartz, president of Society Expeditions, trekking in the Galapagos Islands, officially known as the Archipelago de Colon, would be ''easy.'' ''You should have no trouble,'' he said, adding that people of all ages sign up for Project Galapagos and handle it well.

I discovered for myself that Mr. Schwartz was right. But for the cause of my concern, I refer you to the following passage from Melville, who described these islands in ''The Encantadas'':

''Take five and twenty heaps of cinders dumped here and there in an outside city lot; imagine some of them magnified into mountains, and the vacant lot the sea; and you will have a fit idea of the general aspect . . . a group rather of extinct volcanoes than of isles, looking much as the world at large might after a penal conflagration.''

I've wanted to go to the Galapagos for years - since school days, I suppose, when I got a glimpse, in geography books, of all the weird and wonderful animals and birds that dwell in this island group that straddles the equator some 600 miles off the coast of Ecuador. Yet I knew little more about it than that.

That's where Society Expeditions came in. Mr. Swartz founded the company eight years ago because, he said, ''I thought there had to be people who really wanted to learn something about the places they visited.'' So, in the main, when you sign up for one of Society's ''projects,'' you'll go to one place and study it thoroughly - its culture, its geography, its history, its geology. And before you go, you will be provided with a reading list - in the case of Project Galapagos, some 16 books - so that when you get there you will have a good idea of what you're looking for - and what you're looking at.

In the Galapagos, what you're looking at appears very much as Melville described it. It is a bleak-seeming jumble of craters, caves, and rocks.

Pam and I and the others who made up our group - the two Lindas, Carol, Kristie, Scott, Tressie, Libby, Cathy, and Paul - were following our naturalist-guide Jaap Schoorl on a tramp over Tower Island's lava landscape past frigate birds and boobies - this is the most likely island for seeing the red-footed booby, which uses its bright-red webbed feet to cling to tree branches much like a high-wire walker.

Tower was our second island stop. The Galapagos include 13 major islands, 6 minor islands, and 42 islets that have official names. Scores of smaller rocks and islets dot area waters too. Only a few thousand people live here, and they only on three or so of the islands.

All the main islands have at least two names, one Spanish, the other English. Since the Galapagos Islands belong to Ecuador (they are, in fact, an Ecuador national park), that country's government considers the Spanish names as ''official,'' even though those names are not always the ones more commonly used.

We flew into Baltra from Guayaquil, Ecuador, after doing a few days of touring on the mainland, and immediately transferred to the ship that would be our base for the next week, Galapagos Cruises' Yacht Isabela. City clothes and excess baggage were left in storage at our mainland hotel. With us were the casual shorts, jeans, T-shirts, swimsuits, and sneakers we would need here. In the South American winter season, a sweater would be good, too, and, in both seasons, a sun hat.

There are any number of boats and ships that cruise these waters, some under sail; one, a fullfledged cruise ship that takes some 100 passengers. But Isabela feels just right. At 104 feet, she is small enough to wend her way through shallow waters to reach some islands the bigger boats can't reach, yet she is large enough to give some stability even in the rough sailing months. (From December to June, when the sun is very hot, the seas are calm; from June through November, though the temperatures moderate some, the seas can be quite rough). Plus you have the added advantage of small-group shore trips.

The Isabela has sleeping accommodations for 16 in twin- and four-berth cabins and four bathrooms with showers (no private cabins or facilities). There is a dining room - the meals, which included fresh lobster one night, were basic, but good and varied - a small saloon, an afterdeck with benches and tables and some covering from the sun, and a flying bridge on the upper deck that is just right for watching sunrises or sunsets, or for letting the motion lull your senses.

While it's true that this is a cruise, the word isn't used in the same sense it would be to describe most voyages to the Caribbean or the Mediterranean. It requires somewhat more energy than does donning your long gown or tux to meet the captain.

Every day you will visit at least one island, some days two, and those visits usually entail hikes over lava talus and rocks. While you don't have to be fleet-footed, it certainly helps to be sure-footed. One rainy afternoon, on Santa Cruz Island, we slogged through three miles of ankle-deep mud to see, in their natural habitat, some of the giant land turtles for which these islands are famous.

The turtles, with heads sitting as they do atop long, wrinkled pink-beige necks, rather charmingly resemble E.T., and they, like all the other animals and birds on these islands, are fear-free enough to allow you to get quite close to them for picture taking. You can get so close, in fact, that you had better be quite watchful that you don't step on them.

With the turtles that's quite easy; they are, after all, large and easy to see (although when stationary, they might be mistaken for rocks). However, some of the land iguanas, the lava lizards, and the marine iguanas you will see on Bartolome, Fernandina, and the other islands, blend so well into the black-lava formations that you don't always realize that what you are seeing is living - until it moves.

You have to be equally careful not to step on the birds. It's not that they aren't easily seen. They are. But you are so taken by the fact that you can walk among them without spooking them that you might inadvertently leave the trail and get too close. Especially if you are here when the ground is practically covered with hatchlings. The baby boobies and frigates are fluffy and white and resemble nothing so much as a child's stuffed toy.

The Galapagos is 600 miles west of Ecuador, 800 miles from Panama, some 3,000 miles from any islands to the west of it. This isolation is what makes the Galapagos special.

The islands were formed when lava-spewing volcanoes emerged from the sea millions of years ago. Because the archipelago has never been attached to any land mass, the species of birds, animals, and plants that arrived were able to establish themselves and evolve, often uniquely, on each individual island, adapting to local conditions and forming species endemic to the area. About two-thirds of the resident birds and all reptiles except for one species of gecko are peculiar to the islands.

The most famous visitor here, Charles Darwin, who came in 1835 aboard the British survey ship H. M. S. Beagle, saw ramifications in this circumstance that resulted in the publication nearly 25 years later of his monumental ''On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life.'' His theory, based in large part on the way 13 varieties of local finches have adapted to life here, as seen from the size of their bills and the way they use them, is that life is a process of ''natural selection'' in which organisms gradually adapt to their environments or perish.

Other scientists have visited here since Darwin. Indeed, on Santa Cruz Island , one of our shore trips was to a research center, named after Darwin, which continues to study this unique ecosystem.

But you don't have to have any scientific leanings to enjoy a trip here. It is a photographer's dream and a birder's Eden, a place where you can sit almost within touching distance and watch an hour-old sea lion learn from its mother how to feed, a spot where you can swim in a sun-dappled grotto with some playful fur seals.

Your eyes (and camera) might have to be quick to catch a glimpse of the red-billed tropic bird in flight, but the flightless cormorants, the masked and blue-footed boobies, the swallow-tailed gulls, the sooty terns, the vermilion flycatchers, even a Galapagos hawk stood still and looked right back at us.

This isn't your destination if you're looking for pure, unadulterated comfort (although the crew of Yacht Isabela does better in that department than you'd expect, right down to the warm hors d'oeuvres awaiting you each evening as you ascend the steps from the pangas, the Boston whalers which carry you each day to and from your island jaunts). Come here, and you'll be tired enough after dinner to not miss the night life some other prime vacation spots offer (although we did manage to muster enough stamina to turn the afterdeck into a seagoing disco a couple of nights. Some of the music, borrowed from the Isabela's Ecuadoran crew, had a definite salsa flavor). But it won't take any imagination at all to know you're away from home, to know you're on vacation. Practical information:

For more information on Society Expeditions' Project Galapagos, check with your travel agent or write: Society Expeditions, 723 Broadway East, Seattle, Wash. 98102.

The cost for the cruise is approximately $2,350 a person in a two-berth cabin. Additional costs include airfare to and from Guayaquil and between Guayaquil and the Galapagos Islands.

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