Final answer:
The text structure used in this excerpt is descriptive, as it provides detailed observations and comparisons to paint a vivid picture of the ice formations.
Step-by-step explanation:
The text structure used to organize this excerpt is descriptive. The passage describes the appearance and characteristics of the ice at Swamp Bridge, using detailed observations and comparisons to Gothic city roofs and vessels in a crowded haven. It presents a vivid picture of the ice formations and their relationship to the layers of ice below.