TravelingGeologist

The last stand of the Neptunists – by Siim Sepp

Portrush is a nice little seaside town in Antrim, Northern Ireland which I visited in the early summer of 2012. Portrush is located on top of a dolerite (diabase) sill which forms a promontory and continues off shore in the chain of islands, the Skerries. This outcrop is of major historical significance to geology because Neptunists, who believed that all rocks are sedimentary or crystallized from sea water, used it as an example which should prove that their way of thought is correct.

As we all know now, Neptunists were wrong. Some rocks (igneous and metamorphic) are clearly not crystallized from sea water. But how did neptunists used the sill of Portrush to support their case?

Reverend William Richardson (1740–1820) noticed in the late eighteenth century that there were fossil ammonites in what he thought was basaltic rock in Portrush. So, he reasoned, basalt can not be igneous as Vulcanists, the opponents of Neptunists, believed.

These fossils are indeed ammonites (genus Paltechioceras) and it is not hard to find them even today but the rock which hosts these fossils is not basalt. Reverend Richardson failed to notice or preferred not to see that there is a contact between underlying Portrush sill and metamorphosed (baked) mudstones on top of it which contains the fossils. Eventually, of course, the mistake was acknowledged and Neptunists had been confined to the history books.

When I visited the place I understood that it really is not at all easy to put these puzzle pieces together. The Portrush sill is doleritic (moderately coarse-grained and contains no fossils) but the contact of sill with mudstone is chilled. So it is actually much finer (basaltic) and there is not much difference in the appearance of Portrush sill and the overlying Lias mudstone (now hornfels because of thermal metamorphism caused by the heat of a doleritic magma). Both hornfels and basalt are gray and almost indistinguishable when looked upon only superficially.

Please bear in mind that this place is a conservation area and no attempt should be made to collect specimens by hammering. I did collect some interesting specimens (photos isolated on white) but they were all loose and already somewhat rounded rocks on the coast.

Siim Sepp is a geologist from Estonia. He writes mostly about rocks, minerals, and sand on his blog Sandatlas at www.sandatlas.org

Portrush from the coast.

Layered hornfels (baked mudstone) on top of Portrush sill. Houses of Portrush in the background.

A contact between hornfels (below) and basalt (chilled dolerite) above. Note that this rock sample is upside down. Dolerite sill is actually below hornfels in Portrush.

Fossils of ammonite Paltechioceras in hornfels.

This is Portrush dolerite. Here it is coarse, not chilled. I believe this is how the bulk of the sill looks like. Such dolerite samples are abundant there but outcrop is nowhere to be found. It is most likely below the waterline. Subaerial part of the sill, at least where I saw it, seems to be much finer. Such coarse-grained pebbles are probably thrown on the coast by storm waves.