In "Il portico del foro di Pompei," RM 6 (1891) 168-76, August Mau presents his arguments for dating the limestone colonnade to the period before the 62 earthquake. In reviewing the opinions of previous scholars, he reports that Fiorelli believed that the colonnade was begun in the Augustan period and had remained unfinished in 79, that Nissen considered the colonnade to be post-earthquake, and that he, himself, had thought for a long time that the colonnade was post-earthquake. His reconsideration of the question led to his revised opinion. Mau makes no mention of the presence or absence of a colonnade in front of the Sanctuary of the Genius of Augustus.
Amedeo Maiuri is of the same opinion and states that the substitution of a portico in travertine for an older portico in tufa from the pre-Roman period was begun in the Tiberian period (portico of Eumachia), interrupted by the earthquake of 62, resumed after the earthquake, and still in large part incomplete at the time of the eruption. (A. Maiuri, L'ultima fase edilizia di Pompei, [Rome 1942] 26.)
I hope to continue this discussion and add other published opinions regarding the dating of the forum colonnades. If others can provide this summary or have their own opinion(s) regarding the dating of the tufa or limestone colonnades, I would be happy to provide links here to those Web documents (please send URL). Colleagues who would prefer to have me create the HTML documents and links should send electronic text to firstname.lastname@example.org or hard copy to John J. Dobbins, McIntire Department of Art, Fayerweather Hall, University of Virginia, Charlottesville, VA, USA, 22903. Any such contributions would be identified as yours and would appear under your name.