And all the people of the country of the Gerasenes round about asked him to depart from them, for they were holden with great fear: and he entered into a boat, and returned. (ASV)
This is a geographical error, see Mark 5:13. Also note the unreliability of our version of the Bible, since our oldest manuscripts sometimes have a different name of the town here, such as "Gergesenes", "Gerasenes", etc. --Equinox 2006.08.30
The synoptics have:
1. "...country of the Gergesenes,..." Matthew 8:28
2. "...the country of the Gadarenes..." Mark 5:1
3. "...country of the Gerasenes,..." Luke 8:26
Matthew can identify the people relative to the closet town, Gergesa (the modern Kersa), thus the country of the Gergesenes.
Mark can identify the people relative to the largest nearby city, Gadara, thus the country of the Gadarenes.
Luke can identify the people relative to region, thus the country of the Gerasenes.
In the same way, people living in California may be called Californians, Americans, or westerners depending on the audience to whom one is writing.
This explanation is found in Smith's Bible Dictionary located here 
"These three names are used indiscriminately to designate the place where Jesus healed two demoniacs. The first two are in the Authorized Version. Mt 8:28; Mr 5:1; Lu 8:26 In Gerasenes in place of Gadarenes. The miracle referred to took place, without doubt, near the town of Gergesa, the modern Kersa, close by the eastern shore of the Sea of Galilee, and hence in the country of Gergesenes. But as Gergesa was a small village, and little known, the evangelists, who wrote for more distant readers, spoke of the event as taking place in the country of the Gadarenes, so named from its largest city, Gadara; and this country included the country of the Gergesenes as a state includes a county. The Gerasenes were the people of the district of which Gerasa was the capital. This city was better known than Gadara or Gergesa; indeed in the Roman age no city of Palestine was better known. "It became one of the proudest cities of Syria." It was situated some 30 miles southeast of Gadara, on the borders of Peraea and a little north of the river Jabbok. It is now called Jerash and is a deserted ruin. The district of the Gerasenes probably included that of the Gadarenes; so that the demoniac of Gergesa belonged to the country of the Gadarenes and also to that of the Gerasenes, as the same person may, with equal truth, be said to live in the city or the state, or in the United States. For those near by the local name would be used; but in writing to a distant people, as the Greeks and Romans, the more comprehensive and general name would be given."
The variety of names used is not proof of an error by any of the writers and does not call for that conclusion
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