Silurian effusive basalts and volcaniclastics compose the Svatý Jan volcanic center, which is located in the northwestern limb of the Prague synform, where three major volcanic phases have been recognized: the first one of early to mid-Wenlock and the last of mid-Ludlow age. Two alkaline basalt dikes of late Wenlock to mid-Ludlow age, respectively tilted to the west and to the northeast, as observed in a 100-m-thick tuff sequence, which represents the second volcanic phase, have been extensively sampled. An anisotropy of the magnetic susceptibility (AMS) study of seventy-nine specimens taken from a 5-m-thick dike (dike1) and thirty-two specimens cored...