Leonard D. Harris, a veteran U.S. government geologist, traced his finger along a sloping line on a freshly charted seismic map of central Virginia, displaying a tantalizing clue in an ever-widening search for deeply buried natural gas.

"It's a real frontier," he said. "It'll prompt a lot of people to look very carefully in the area to see if it's worthwhile to lease it and drill." What Harris' map disclosed was a hidden rock layer, thousands of feet underground, that stretches from Staunton across the Shenandoah Valley to Charlottesville about 100 miles southwest of Washington--a possible new target for gas exploration in Virginia.

Amid such promising signs, gas and oil company geologists, seismic crews and land-leasing specialists known as land men have crisscrossed Virginia, West Virginia and many other eastern states, seeking leads to possible gas and oil deposits concealed within complex geological formations known as the Eastern Overthrust Belt.

In Virginia alone, acreage leased for exploration by petroleum and gas companies has more than tripled since the rush started in 1977, reaching almost 3.4 million acres last year, more than one-eighth of the state's total area. State officials optimistically predict that the Old Dominion, now a minuscule factor in American oil and gas production, may be propelled into the major leagues.

"The Appalachians are one of the largest untested regions in the world," notes state geologist Robert C. Milici. "What we need is one good well. It would just blow it wide open."

While oil and gas drilling once was confined chiefly to small independent companies operating in the southwestern corner of Virginia, today major U.S. petroleum corporations hold exploration rights to sizable blocks of land extending from the southwest to the Shenandoah Valley and into the Piedmont.

Amoco Production Co., the exploration arm of Standard Oil Co. (Indiana), has the largest Virginia holding--rights to 554,805 acres, according to state figures. Rights to hundreds of thousands of acres also are leased by Exxon Corp., Gulf Oil Corp., Sun Co. and other oil giants. Farmers and other land owners usually get only nominal $1-an-acre yearly fees for the leases, sometimes boosted by small bonus payments. But they stand to draw big royalties, amounting to one-eighth of the proceeds, if gas is produced.

Texas-based seismology firms have completed thousands of miles of computerized probes, at prices recently reaching $5,000 a mile or higher, in Virginia and other states in the Overthrust Belt, an arc extending from New York and Vermont to Georgia and Alabama. Millions of dollars have already been spent on exploratory drilling, including several wells in Virginia's Shenandoah Valley.

With most recent seismic findings held as tightly guarded industrial secrets, Harris' new geophysical maps offer an unusual public glimpse into subterranean prospects. Spread across a work bench at U.S. Geological Survey headquarters at Reston, Harris' cross-sectional diagrams and seismic profiles reveal a hidden layer of gas-prone sandstone, shale, limestone and dolomite, a rock chemically similar to limestone. Harris estimates it lies 7,000 to 10,000 feet underground near Staunton and dips to more than15,000 feet at Charlottesville.

"This kind of data is the first time you can really see what the subsurface relationships are," says Harris, whose earlier predictions of extensive hidden sedimentary layers--rock formed from erosion, decomposition and rehardening of other rock and decaying organisms--helped spur the initial corporate rush to get a foothold in the Eastern Overthrust in the late 1970s. The new data reconfirm Harris' original forecast.

To probe beneath Virginia, Geological Survey scientists used seismological techniques that have already revolutionized oil and gas exploration. They hired a Dallas-based firm, Geophysical Service Inc., a subsidiary of Texas Instruments Inc., to conduct $600,000 worth of seismic testing from Staunton along Rte. I-64 to Richmond and then to Hampton in the Tidewater area.

With truckloads of electronic equipment, seismic crews monitored sound waves as they bounced off different underground rock layers. The waves were set off by huge diesel-powered vibrating machines and detected by electronic devices called geophones, strung along the highway for two miles at a time.

By timing the waves' travel, scientists can determine their velocity and use this as an index to identify rock layers. Different rocks display characteristic velocities. The waves' echoes were recorded on electromagnetic tapes and unscrambled by computers, producing squiggly seismic profiles still being analyzed by Harris and his Geological Survey colleagues.

While the sedimentary layer extending eastward from Staunton may hold clues for gas exploration, federal researchers also turned up novel seismic findings that may suggest new ways to search for buried metal ores or challenge much-debated theories about the geologic creation of the Appalachian Mountains.

The Appalachians, according to some current geological thinking, may have been formed hundreds of millions of years ago--long before the creation of the present Atlantic Ocean--as a result of a collision between the African continent and the eastern edge of the North American continent, a vast convergence that may also have encompassed minicontinents once lying between the American and African plates.

But Harris pointedly questions this thesis. "We don't see that in Virginia. We see a continuation of a big sheet system," he says. "We're right at that stage where things are beginning to look different." The new seismic data, Harris says, indicate that rock formations underground resemble those thrust over them from the east. So all the Appalachian rocks may have originated in the American continent, not in an ancient African continent or minicontinent.

"It looks like it's part of the same basin," Harris says. "Nothing came from Africa. It's all just part of this continent."

Harris suggests the new seismic study may also hold promise for metals exploration. It demonstrates for the first time, he says, that seismology can distinguish between different layers of metamorphic rock--rock formed under great pressures and heat. Some deeply buried metamorphic rock, geologists note, contain significant deposits of metals, such as zinc and copper, which might someday be extracted after seismic probing.

Pinpointing a sedimentary layer, like the one beneath Staunton, is only a first step in searching for gas, Harris and others are quick to point out. More detailed seismic work and costly drilling would be needed--a high-risk gamble for exploration companies. Yet the Geological Survey's new charts clearly show how far east the lure of the Overthrust Belt may extend.

The 1,100-mile formation got its name because ancient metamorphic and volcanic rock was thrust from the east by powerful forces eons ago over younger rock lying to the west, forming complex faults and folds among intertwining layers. To track possible gas or oil deposits deep within such formations is virtually impossible without skillful seismic scrutiny.

Skyrocketing oil prices and gradual gas deregulation, along with advances in seismic and drilling techniques, spurred exploration in the Eastern Overthrust, particularly after Amoco struck gas in Pennsylvania in 1977. Columbia Gas Transmission Corp., which supplies most of the Washington area's gas, found sizable Overthrust gas deposits in West Virginia in 1979. Geological Survey and Cornell University research added impetus, as did an oil and gas boom in the Western Overthrust Belt, an analogous though geologically younger formation in the Rocky Mountains.

Major companies are moving to start exploratory drilling at new Eastern Overthrust sites. Corporate spokesmen term it a glamor region. And the Oil and Gas Journal, an industry publication, predicts that "the potential exists for the Eastern Overthrust to rival the Western Overthrust."

In the East, geologists say, Overthrust exploration will focus mainly on gas, rather than oil, because high temperatures resulting from deep burial would likely have cooked any petroleum. Oil changes to gas at around 194 to 284 degrees Fahrenheit. Fractures in rock layers, caused by the overthrusting, may create space for gas deposits to accumulate.

Some drilling is under way in Virginia and other Overthrust states, although geologists say exploration temporarily slowed recently because of tighter oil-company budgets stemming from declines in world crude prices in 1981 and early this year. "Potential projects are now just waiting for the economy," says Virginia oil and gas inspector Tom Fulmer, predicting a resurgence by late summer. "We've got thousands of acres yet to be drilled."

Atlantic Richfield Co. recently sank a 12,400-foot gas exploration well, reportedly costing $6.5 million, in Lee County at Virginia's southwestern tip, an area that has historically produced tiny amounts of oil and virtually no gas. The company has yet to announce results. Last year, Amoco drilled 9,688 feet near Winchester and 6,620 feet not far from Harrisonburg in the Shenandoah Valley, but abandoned both as dry holes. Columbia Gas went down 9,678 feet in Botetourt County north of Roanoke without hitting significant deposits.

"Although it was dry, we learned a lot and hope to use that in the near future," says chief Columbia geologist Porter Brown.