Entry in progress—B.P.
 
Wikipedia: Dallas
Dallas /ˈdæləs/ is the ninth-largest city in the United States and the third-largest city in the state of Texas. The bulk of the city is in Dallas County, of which it is the county seat. However, slices of the city are located in Collin, Denton, Kaufman, and Rockwall counties. The city had a population of 1,197,816 in 2010, according to the United States Census Bureau.
 
8 May 1910, Dallas (TX) Morning News, pg. 5, col. 1:
DALLAS, “THE CITY OF THE HOUR.”
Such Is Prize Slogan Adopted by the Chamber of Commerce, “The Pulse of the State.”
Dallas now has a new slogan, or to be more precise, three slogans. Some weeks ago the real estate firm of Murphy & Bolanz offered $50 to be divided into three prizes for the best three slogans submitted, competition open to the whole world. More than 600 persons tried for the money, about 200 of them being nonresidents. Two weeks ago, the directors of the Chamber of Commerce appointed a special committee to go over the mottoes submitted and award the prizes. The committee put in more than a week on the task.
 
The first prize of $35 was awarded to Samuel Rizzotto of Dallas, and his motto is “The City of the Hour.”
 
The second prize of $15 was won by Fred J. Lagler, also of Dallas, and his suggestion was “The Pulse of the State.”
 
The third prize of $10 has been divided between Madeline Frees of St. Louis, formerly a resident of Dallas, and Mrs. L. V. Craig of Dallas, they both having selected the same, “Dallas on Top.”
 
Google Books
The World
1916 Almanac and Encyclopedia
New York, NY: The Press Publishing Company
1914
Pg. 772:
SOBRIQUETS OR BY-NAMES OF CITIES IN THE UNITED STATES.
(...)
Dallas—The City of the Hour.