The Granite Controversy: Geologists pilgrims

Minerals found in the valley of Fassa were requested trophies for the various cabinets of curiosities in Europe and especially Italy during the 18th century.

Fig.1. Fassaite is a variety of augite but having a very low iron content. It is named after the Fassa Valley. It is thought to be a contact mineral formed at high pressure on the interface between volcanic rocks and limestone.

However the first publication dealing specifically with the geology of the Val di Fassa was the monographic work by G.B. Brocchi in 1811 "Memoria mineralogica sulla Valle di Fassa." The publication was a great success, and soon the book became translated in the most important scientific languages of the time: German and French.

In 1820 Count Giuseppe Marzari-Pencati published his observations of the stratigraphic succession near the village of Predazzo in the Fassa Valley, at the site of Canzoccoli. He recognized the superimposion of granitic rocks onto sedimentary marbles - a geological impossibility for the geological concepts of Neptunism.

Fig.2. The locality of the Canzoccoli today, the withe outcrops are composed of "Predazzite."

Fig.3. Example of Predazzite - the peculiar limestone found in contact with the granite is today regarded as contact-metamorphic limestone - common marble.

Fig.4. Interpretative sketch of the situation of the Canzoccoli by a geologist in 1849, the marble is covered in part by a "granite" (Monzonite), but the marble itself covers the granite - contradicting the notion of the Neptunism theory of granite as unique basement of all other rocks.

In the year 1822 one of the most important geologists of the time, and scholar of Werner, the German Leopold von Buch visited the village of Predazzo to find a Neptunism explanation of the described stratigraphic order. In a first attempt he interpreted the geological situation as a result of a large landslide, which disturbed the natural order of the rocks. However the limestone displayed a aureole of contact metamorphism caused by the hot granite, so the limestone in fact was older and in situ, and later transformed in marl by the intrusion of the granite.
During a second visit in September 1822 Von Buch was accompanied by one of the greatest naturalist of the time, also scholar of Werner, the geographer Alexander von Humboldt.
But Neptunism is already a dying theory...

Fig.5. Many prominent geologists of the 19th century visited the village of Predazzo to study the peculiar geology.

Bibliography:

AVANZINI, M. & WACHTLER, M. (1999): Dolomiti La storia di una scoperta. Athesia S.a.r.l. Bolzano: 150
DELLANTONIO, E. (1996): Geologia delle Valli di Fiemme e Fassa. Museo Civico " Geologia e Etnografia" Predazzo: 72