I agree with Mrs. Campbell's post. The tone of "The Brain is wider than the sky" is contemplative and revelatory. The speaker has discovered that the brain is capable of infinite things... even the creation of God. The speaker is contemplating metaphysically what the brain can do. It is wider than the sky, it is deeper than the ocean, it is the weight of God. The speaker is in an ultra-contemplative state. She marvels at the complexity of the human brain and goes so far as to begin to challenge the existence of God as anywhere but in our own minds. You can decide if she believes that God resides within our brains or if we reside within God's. The poem contains a deeply philosophical argument and philosophical tone.
Tone in literature and poetry usually refers to the emotion, feeling and perspective that comes through behind the words. Word choice and arrangement alter the tone greatly, and Emily Dickinson, because she chooses her words so carefully, is able to create very distinct feelings and tones in her poems. In this poem, whose point is to indicate that our brains are powerful sources of imagination, knowledge and scope, conveys a tone of awe, contemplation and reverence. As you read the poem, it is easy to feel that Emily herself is held in wonder of the capacity of our minds. She shows this sense and tone of wonder by comparing the brain to very huge things--the sky, the sea, and even God himself. Comparing our minds to such huge, vast and powerful things definitely sets a tone of wonderment and awe. She is reveling in how amazing our minds are, and to do so, picks these very large things to compare them to.
Along with that feeling of awe and wonderment is one of happiness and joy. She uses bright, happy colors like "Blue to Blue", sweet and soothing sounds and references like "Syllable from Sound," and energetic and optimistic words like "Heft" and "Buckets", all that enhance the feeling of happiness and strength of the tone. All in all, Dickinson seems to be rejoicing and revering our minds, and both of those feelings come through in her poem. I hope that helped; good luck!