Suburban Bancorporation, a big bank holding company based in Hyattsville, joined a host of local or regional banking corporations yesterday in reporting increased earnings during 1979.

Although Bank of Virginia Co. bucked the trend in reporting lower profits for the final quarter of the year, all of the financial companies listed substantial gains for the year and most showed more modest fourth-quarter increases because of the government's tighter money policies.

Suburban Chairman Robert Tardio, in an assessment of earnings gains last year that also reflects business trends at other institutions, said there were higher average assets on which the company earned money and an increase in net interest income, after expenses.

Earning assets for Suburban rose 14 percent to $1.2 billion while net interest income was up 19 1/2 percent to $66.6 million.

Not counting securities transactions, Suburban profits for the year were $16.2 million ($3.44 a share) compared with $13.4 million ($2.85) in 1978. Fourth-quarter oprating profits rose to $3.8 million (80 cents) from $3.6 million (75 cents).

Average consumer loan volume, including credit card transactions, rose by 34 percent and real estate loans increased by 29 percent, Tardio reported. Total deposits expanded by $100 million to $1.2 billion. With assets of $1.4 billion, Suburban operates more than 70 banking offices throughout the state.

Bank of Virginia Co., a Richmond bank holding firm, said operating earnings last year rose to $14.9 million ($3.15) vs. $13.6 million ($2.88) in 1978 while fourth-quarter earnings dipped to $3.6 million (72 cents) from $3.8 million (79 cents).

First & Merchants Corp., another Richmond bank holding company, said operating profits last year rose to $15.2 million ($4.58) from $11.76 million ($3.57).

Dominion Bankshares Corp., of Roanoke, said 1979 operating income was $19.7 million ($2.82) vs. $16.6 million ($2.39) the previous year.

Madison National Bank, of Washington, reported 1979 net income of $2 million ($5.12 a share) compared with $1.5 million ($3.78) the prior year, as assets climbed 23 percent to $158 million.

D.C. National Bank said operating income rose to $1.16 million ($5.65 a share) from $689,181 ($3.45) in 1978, with assets up 17 percent to $106 million.

Two savings and loan companies -- First American of Woodbridge and Baltimore Federal -- reported mixed results for a year during which the thrift industry's profits were reduced sharply by soaring interest rates.

First American earnings rose moderately to $922,158 ($3.06 a share) from $873.031 ($2.89) as assets increased 17 percent to $103 million and savings deposits gained 25 percent to $78 million.

Baltimore Federal earnings declined to $5 million from $6.3 million but assets rose 7 percent to top $1 billion for the first time.

Computer Data Systems Inc., a professional and processing services firm based in Bethesda, reported earnings from continuing operations for the six months ended Dec. 31 rose to $288,400 (52 cents a share) from $179,300 (33 cents). Net income from discontinued operations contributed an additional 3 cents a share in the recent period while sales almost doubled to $6.7 million.

CACI Inc., a professional research and services firm in Arlington, said earnings for the six months ended Dec. 31 rose to $791,269 (77 cents) from $453,482 (44 cents) as sales increased 73 percent to $14.2 million. On an annual basis. CACI's average return on equity rose to 39.4 percent in the 1979 period from 31.2 percent a year ago.