The first part of this paper (which accompanied a series of geological specimens from Auvergne, presented to the Society) relates to several specific points which tend to assimilate the evidence for the igneous origin of trap-rocks generally, with that afforded by the volcanic district of Central France. The altered character of the stratified deposits with which igneous rocks have been intermixed, is one of their most striking features; yet we occasionally find cases where this evidence is far from being so obvious as might be expected; and this dubious character, which is particularly remarked in the hill of Gergovia, near Clermont, forms an admirable parallel to some cases in trap districts where a like want of alteration occurs.