This study was conducted primarily to clear up the stratigrapby in a lowgrade metamorphosed and fossiliferous area in the Trondheim region, and to get a safe foundation for further investigations in more metamorphosed parts of the same region. As to facies development, the Hølonda-Horg area occupies about the same position in the Norwegian parts of the Caledonids as the Girvan area and vicinity in the Scotch parts. Three conglomerates, representing as many breaks, divide the Ordovician to lowermost Silurian series of strata into four parts, viz. the Støren, Lower Hovin, Upper Hovin and Horg Series. These conglomerates can be traced in various areas in the Trondheim region and in Western Norway. The break between the two Hovin Series, representing the Ekne Disturbance, close to the divide between the Caradocian and Ashgillian Series, seems to be of greatest general importance. Besides the well known effusive Støren Greenstones of Skiddavian age three younger divisions of volcanic rocks are established with more or less certainty, probably ranging from Llandeilian to Ashgillian. The volcanic series, comprising basalts, andesites and rhyolites, exhibits a decided prevalence of soda as compared to potash. The younger volcanic rocks accentuate the analogy between the British and Norwegian parts of the Caledonids.
The geology of part of the Hølonda - Horg district, a type area in the Trondheim Region.