What on Earth are you doing with Howard Bannister’s rocks?


The classification of the igneous rocks is a morass from which you would be well advised to steer clear. Even Judy Maxwell said, “I can take your igneous rocks or leave them. I relate primarily to micas, quartz, feldspar. You can keep your pyroxenes, magnetites, and coarse-grained plutonics as far as I’m concerned.”

Personally, I love the igneous rocks. Nevertheless, there is one coarse-grained plutonic up in the mountains above Santa Fe which has given me fits in trying to classify. And it points perfectly to the sort of look-alike confusion which plagues the field identification of these rocks.

Here’s the rock:

The speckled rock along Tesuque Creek

The speckled rock along Tesuque Creek

Ideal countertop material, you might say. I asked a hiking companion what he thought it was and was told “it looks just like the granite back home up in the Sierra” – the Sierra being the Sierra Nevada Mountains in California. And it does look just like those granites, except for the caveat going though my head – the curse of an education – that, as the geologist P.B. King relates, in the Sierra Nevada, “true granites in the technical sense are rather minor, most of them being the somewhat more mafic quartz monzonites, granodiorites, and quartz diorites”.

I thought it might be diorite. Diorite is an interesting construction, a French name built from the Greek root dior izein, ‘to distinguish’. Diorite is a granular igneous rock made up of bright white feldspar and dull black hornblende, with a classic “salt and pepper” appearance that every first year geology student learns to identify on sight.

Unfortunately, diorite is very difficult to distinguish from gabbro, another dark speckled igneous rock, which is what another hiking companion (understandably) always thought it was.

There’s a reason field geologists carry around that little ten-power hand lens, and when you look at this rock up close, you discover that most of the dark minerals are the black mica called biotite, and that there is an awful lot of quartz mixed in with the white feldspar. This throws the ball back into granite’s court, petrologically speaking, and there is a surprising name for the common hybrid between granite and diorite: granodiorite. So that’s where I finally decided to pigeonhole the rock. A very dark granodiorite.

Except that I found out its real name is tonalite.

The point is, the point is… oh god, I’ve forgotten my point.