They reached Horta early on 21 (not 20) June. Like most of the other passengers, Clemens spent a busy day on shore and probably began this letter when he returned to the ship that evening. Continued rough seas delayed departure for Gibraltar until noon of 23 June.

SLC to Jane Lampton Clemens and Family, 21 and 29 June; 1, 3, and 5 July 1867, Azores, Portugal (UCCL 00139), n. 1. 

The Azores are a group of volcanic islands west of Portugal.  The Quaker City was to land at San Miguel, the largest of the islands, but a storm caused them to make port at Horta on the island of Fayal.  Rather than visit San Miguel the Quaker City then proceeded to Gibraltar.