Written In Stone

By R.Z. Sheppard

TITLE: ASSEMBLING CALIFORNIA

AUTHOR: JOHN McPHEE

PUBLISHER: FARRAR, STRAUS & GIROUX; 304 PAGES; $21

THE BOTTOM LINE: A peripatetic author proves that geology can be a page turner.

ASSEMBLING CALIFORNIA IS THE fourth and final volume of John McPhee's "Annals of the Former World" series, his reports on the new geology that began running in the New Yorker back in the Shawnian Era. That was before the Albion Shift uplifted Tina Brown from London and thrust her toward Manhattan, where she eventually docked as the magazine's new editor.

McPhee concluded his project at a good time. Brown's interests are topical, not topological. Even dedicated subscribers to the old New Yorker can be forgiven if their eyes lost traction on McPhee's exotic terrain and skidded to the cartoons. Those who stayed with "Annals" soon learned to appreciate the enterprise. McPhee is a master of expository prose.

A lesser writer could not have made a subject as abstruse as plate tectonics both intelligible and readable. Seen in four dimensions -- not least of which is time -- the earth appears as an endless, slow-motion demolition derby. Untangling cause and effect challenges both mind and imagination. Nature has had 4 billion years to jumble the record, and geologists not even two generations to begin to grasp the mechanisms of continental drift.

So far, the evidence suggests that earthquakes are incremental steps in the movement of the 20 or so lithospheric plates that make up the planet's crust. It takes about 50,000 major jolts to nudge a plate 100 miles. Eldridge Moores, the geologist who guides McPhee, believes California was formed when a 2,000- mile-long arc of land parked against North America 250 million years ago.

How does he know? Moores is a specialist in ocean crust, which he routinely discovers high and dry in the Sierra Nevada. "The whole vast assemblage of transported deep-ocean rock," writes McPhee, "now rests on California like a ship stuck in sand, listing thirty degrees to the west." Scientist and writer poke through the wreckage. They straddle fault lines and sift through road cuts while impatient drivers speed by.

It is soon apparent that Moores and McPhee share more than a passion for knowledge. Geology is a way for both men to get out of the house. From California they fly to Cyprus, still a tectonic mystery, and to Greece, a microplate caught in a squeeze between Africa and Europe.

Travel suits the peripatetic nature of the narrative. The world is an agora through which Moores and McPhee amble and learn. Whenever their exchanges seem about to burst with an excess of ophiolites, abyssoliths and subduction zones, McPhee relieves the pressure with anecdotes and historical nuggets. Included is a tectonic dish of special interest to Californians. During the past 2,000 years, part of the San Andreas Fault near Los Angeles has been wrenched by 12 major earthquakes. On average they occurred 145 years apart. The most recent Big One hit in 1857. McPhee makes no predictions but figures his readers are smart enough to do their own arithmetic.