Galloway's purpose in writing the letter was to point out his solution to the problem of colonists resisting the imposition of taxes by the British government. It was the first impulse of what would later be known as the Galloway's Plan of Union, which consisted in creating a parliament of American colonists who would oversee the decisions the British parliament had for the new continent.
In the letter, Galloway recognized the burden of sustaining the colonies as being a reasonable cause for the United Kingdom to tax colonists, but recognized the inability of the later to pay due to the trading restrictions set by the English.
He suggested that if colonists considered the impositions too heavy, they should refer to more rational methods than rioting to let the British understand that they couldn't pay the taxes, for example by sending American representatives to the British parliament.
One way to get extra information that might help you determine the relative ages of these rocks is to examine any fossils contained in the layers. The time period that these fossils appear might be known, and this could help you begin to sort out which rock layers are older than others.
