These features could made by rock-boring molluscs such as Pholas

Ian,

Thank you for your contribution! Sounds like a plausible explanation. That implies that the original rock could have been a limestone and was subsequently altered to silica, molecule by molecule, leaving the original borings intact.

Thank you again very much!

Hi,

I do not support  the hypothesis of Ian. Gastrochaenolites, the name of the boring in Ichnology science are different.

Please Michael, provide others pictures and information on landscape, geological setting, location.

Cheers

Jerome,

Unfortunately, I have no other information as this rock has been transported by glaciers and is far from its original location.

Thanks for your contribution indeed.

The "vesicles" look like trace fossils, e.g. worm tubes, maybe root casts, but they don't branch as you would expect for root casts. 

Brian,

Thank you for your input.

I will try to break it up to see if the tubes continue inside the rock.

I originally thought it was a pillow lava, that is why I picked it up. Then it turned out it was  a silicified rock.

Hi Jerome

It is certainly an interesting erratic. My guess is that it could be a granophyre arrested at the eutectic during migration of a dispersed immiscible basic melt.  A thin section in a petrographic microscope will provide a definite answer.

That is an interesting suggestion and I would follow it up.

Thank you Norman.

Examining erratics is not my past time, but I have been looking in boulders piles trying to see if one can get rare rocks like purple porphyry in North America. In ancient and medieval times purple porphyry was the most sought after construction stone for statues, columns, etc. There was only one place to find it: an impossible location in the desert in Egypt. Maybe it can be found elsewhere today.

The rock is a limestone with some type of coral (Syringopora?) that is cut so you are looking down on it.  I do not see any kind of glacial striations on the rock, but could have easily been transported by a glacier.  What part of the country is the sample from?

Good luck,

Paul Grover

Paul,

The sample was found on the lower slopes of Porcupine Mountain (a moraine) in southwestern part of Manitoba province around the 52nd parallel.

Thank you for your contribution.

Dear Michel,

what an interesting rock! I vote for silicified (mildly metamorphic) spilitic pillow lava, where the vesicles represent bubbles of gas that exolve from the molten basalt under the cooling crust, immediately before the entire pillow becames solid.

I have once found a small round pillow of pillow lava from the Jurassic Ophiolitic series of the Norhern Apennines (Italy) during the field work for my thesis at Pisa University (1977-1978), and I had it cut to obtain four book-ends for my library. Still enjoing them, after so many years.

Best regards

Roberto

Hello Mark,

Thank you for your contribution.

I found the Skolithos sandstone a very interesting and unique formation. I have not seen or described in my area, but that doesn't mean is not there. The Cambrian formations are limited in extent and very rare in outcrop. 

In Virginia (Appalachians in USA), however,  Wikipedia reports that Skolithos is the "oldest known trace fossil".

This location however, is far down-ice from my location and does not apply for my boulder.

Thanks again for your input.

Hello Roberto,

Your suggestion may be right on account of the green colour of the rock. I still need to work out the hardness of the rock. It could be albite (not quartz) that makes the rock "silicified" and is typical of spilites.

I wonder is your pillow lava (book-ends) green in colour and is this one a spilite, too?

One problem with the cavities (gas bubbles) is that some of them look like long pipes (like worm holes) but this long dimension could be the result of flattening of the rock by compression during mountain -building for example.

I am still stopping in bridges and looking at the big rocks below to see what I could find next..

Thanks again for your contribution!

Hi Michael,

nice to correspond again with you.

Yes, my  pillow lava is greenish and it is classified as Spilite (also Diabase, European usage).

Please have a look at my 1978 publication on pillow lavas in Northern Apennines (attached). It contains, among the rest, also tables of mineralogical and chemical composition of the pillow lava itself.

The publication focussed on "strange" cavities filled with ocean-bottom radiolarian mud, with turned out to represent the space left in the top part of a pillow when the crust is broken elsewhere and molten lava partially pours out of the pillow. The radiolarian mud was sucked in most of the times inside the cavity.

An interesting way to find a precise "way-up" in structural geology.

Warm regards

Roberto

In my previous response I was attracted to the igneous possibility by Michael's comment that it could not be scratched by steel. I'v been having second thoughts influence d by mark kiter's mention of Scandinavian Scolithus sandstone. After a second look at Michael's fourth photo I noticed a chain-like organism in the bottom right of the photo.

This prompted recall of another erratic I found on the beach at Narin, Co Donegal a few years ago. This rock is almost certainly from the adjacent Dalradian Falcarragh Limestone formation. I attach two photos of this erratic boulder. The first is a close-up of the second, showing a similar chain-like structure. This limestone is metamorphosed and contains lamellar calcite intrusions.  Calcite and limestone of course can be scratched by steel.

Here's two more photos. What is this apparent organism? My previous image 1871 also has dark brownish clusters. Could this be recrystallised dolomite?

Hello Roberto,

Thanks for the wealth of information you have provided.

I found your publication very interesting, especially the fact that quartz or "siliceo" seems to be there, although the information on spilites, I found, does not mention quartz was one of the minerals present.

I saw in your drawings of the pillows a set of prominent linear cavities as well as a set of smaller "holes" near the periphery of each pillow. I wonder if my "pillow" has just these peripheral cavities, not the prominent linear ones.

I turned my pillow on its side and saw that, at that angle, the cavities seem to be forming an arc, as if they were found just inside the periphery of the pillow.

Thanks again for your contribution. I think we are getting near the bottom of the story in this way.

Best regards,

Michael

Hello Norman,

You have a very interesting boulder and the pictures show the structures very well.

To me they look like they were made by some kind of organism, but I am not very good on structures and diagenetic processes. There is a definite resemblage to the skolithos rocks.

Hope another expert will have an opinion.

Best regards,

Michael