All 6 Uses

solemn
in
The Sign of the Beaver
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To Matt's astonishment he spoke to the fish quite solemnly, a few incomprehensible words, then tossed it back into the stream.†
p. 46.9 *

solemnly = with seriousness and dignity
Matt knew that Attean was mocking when some of his own favorite expressions came solemnly out of the Indian's mouth.†
p. 67.3
He stood over the bear, and he began to speak, slowly and solemnly, in his own tongue.†
p. 73.9
Attean looked at him, and his solemnness suddenly dissolved in a grin.†
p. 74.3

solemnness = seriousness
The Indian boy was solemn and unsmiling, looking more like his grandfather than ever before.†
p. 106.5

solemn = very serious--possibly dignified
His solemn face made Matt's heart sink still lower.†
p. 110.5

Definitions:

(1) (solemn)
in a very serious (and often dignified) manner
(2) (meaning too rare to warrant focus)
Less commonly, solemn can mean that something was done with great or appropriate ceremony. It can also be used to describe something as dark or undecorated.