Question: Stone Mountain at it's longest point continues into which county?
About the article "Stone Mountain", section "Geology".

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Stone Mountain is a pluton, a type of igneous intrusion.
Primarily composed of quartz monzonite, the dome of Stone Mountain was formed during the formation of the Blue Ridge Mountains, part of the Appalachian Mountains.
It formed as a result of the upwelling of magma from within the Earth's crust.
This magma solidified to form granite within the crust five to ten miles below the surface.
Stone Mountain pluton continues underground at its longest point into Gwinnett County.
Numerous reference books and Georgia literature have dubbed Stone Mountain as “the largest exposed piece of granite in the world".
This misnomer is most likely a result of advertisement by granite companies and early park administration.
Stone Mountain, though often called a pink granite dome, actually ranges in composition from quartz monzonite to granite and granodiorite.
The minerals within the rock include quartz, plagioclase feldspar, microcline and muscovite, with smaller amounts of biotite and tourmaline.
The tourmaline is mostly black in color, and the majority of it exists as optically continuous skeletal crystals, but much larger, euhedral pegmatitic tourmaline crystals can also be found in the mountain's numerous, cross-cutting felsic dikes.
Embedded in the granite are xenoliths or pieces of foreign rocks entrained in the magma.
The granite intruded into the metamorphic rocks of the Piedmont region during the last stages of the Alleghenian Orogeny, which was the time when North America and North Africa collided.
Over time, erosion eventually exposed the present mountain of more resistant igneous rock.
This intrusion of granite also gave rise to Panola Mountain in DeKalb County, smaller outcroppings further east of Stone Mountain.
Hwy 78 enters Georgia in Haralson County, and then proceeds through Carroll County and Douglas County.
In Douglasville, located in Douglas County, Hwy 78 runs through the downtown, historical part of the city.
It is the original thoroughfare for these Georgia counties.
There is another short freeway portion east of Atlanta, leading from just inside (west of) I-285 (the Perimeter) eastward to the suburb of Stone Mountain, Georgia.
This portion is named the Stone Mountain Freeway, and has an excellent view of Stone Mountain while traveling eastbound.
The route then continues eastward through DeKalb, Gwinnett, Walton, and Oconee counties.
In Oconee county US 78 leaves Moina Michael Highway at the Georgia 316 interchange, turning right and running concurrent with Ga. 316/US 29.
(From this point, Moina Michael Highway is signed as US 78 Business, which follows the original route of US 78 through Athens, Ga.) At the terminal eastern interchange of Ga. 316 both US 78 and US 29 turn right and join with Ga. 10 Loop, a mostly interstate-grade bypass that rings Athens-Clarke County.
US 78 exits the bypass and turns right at the Lexington Road interchange.
From there US 78 passes through Oglethorpe, Wilkes, McDuffie and Columbia Counties into Augusta and then onto one of the twin bridges across the Savannah River into South Carolina. 
At the top of the mountain, PA 305 crosses into Union Township in Mifflin County and leaving the boundaries of Rothrock State Forest and beginning to descend the forested mountain, curving to the east.
At the base of Stone Mountain, the road turns to the southeast and heads into the Kishacoquillas Valley, passing areas of agriculture with some residences.
PA 305 continues southeast to its eastern terminus at an intersection with PA 655 near the community of Belleville. 
The southern end of Sleepy Creek Mountain, Shockeys Knob (1,873 feet/571 m), lies in northern Frederick County, Virginia and is shared by Berkeley and Morgan Counties in West Virginia.
The mountain continues along southwest-northeast orientation between the two counties and peaks again at High Rock (1,811 feet/552 m).
Sleepy Creek Mountain is conjoined with Third Hill Mountain at "Locks-of-the-Mountain".
From "Locks-of-the-Mountain", the Meadow Branch of Sleepy Creek is formed flowing northward between the two mountain ridges.
North of this point, Sleepy Creek Mountain reaches its highest summit at 1,905 feet (580 m).
East of Johnsons Mill, Whites Gap (1,492 feet/454 m) is formed in mountain.
With Sleepy Creek on its western flank, and Meadow Branch to its eastern flank, Sleepy Creek Mountain is then located entirely in Morgan County.
Sleepy Creek Mountain's northern end lies at the confluence of Sleepy Creek and its Meadow Branch.
Just south of the its northern terminus, on the eastern flank is the Devils Nose Rock formation (not to be confused with the nearby Devil's Nose Mountain), which juts out into the Meadow Branch gorge forcing a sharp bend in the creek. 
Shelburne Moriah Mountain is a mountain located in Coos County, New Hampshire.
The mountain is part of the Carter-Moriah Range of the White Mountains, which runs along the northern east side of Pinkham Notch.
Shelburne Moriah is flanked to the southwest by Middle Moriah Mountain.
It is the northernmost Moriah Range summit along the Appalachian Trail, which crosses the Androscoggin River at Shelburne, New Hampshire, and continues north into the Mahoosuc Range.