Does indexing help >= as well as = for integer columns?

I have a table of about 5 million rows, 24 columns.
Integer column _c is BTREE indexed (as is _n, _o and 3 others).

This I understand and like:
Explain Analyze Select count(smiles) from structure where _c = 30
Aggregate  (cost=105595.11..105595.11 rows=1 width=32) (actual time=17.722..17.724 rows=1 loops=1)
  ->  Index Scan using "Nc" on structure  (cost=0.00..105528.89 rows=26486 width=32) (actual
time=0.098..16.095 rows=734 loops=1)
        Index Cond: (_c = 30)
Total runtime: 18.019 ms

This I don't get.  Why is an index scan not used?  Isn't an index supposed
to help when using > < >= <= too?
Explain Analyze Select count(smiles) from structure where _c >= 30
Aggregate  (cost=196033.74..196033.74 rows=1 width=32) (actual time=42133.432..42133.434 rows=1
loops=1)
  ->  Seq Scan on structure  (cost=0.00..191619.56 rows=1765669 width=32) (actual
time=8050.437..42117.062 rows=1569 loops=1)
        Filter: (_c >= 30)
Total runtime: 42133.746 ms


TJ