"Why should it form in a regular thread like this?"
"You mean a vein? Hard to tell precisely. Varying rates of water seepage, varying degrees of oxygen or carbon dioxide penetration, varying degrees of compactness in the rock where the stuff is formed—"
"I don't mean a vein. This is in a cylindrical body going right through the core from one side to the other, as though there had been a copper wire there originally which had been attacked by soil acids."
"Let's see. You're right. It's hardly an ordinary vein, though your suggestion seems a trifle far fetched. The paleontologists can probably furnish an idea. Maybe a vine or even a worm buried in the mud flow acted as the precipitating agent for copper salts in the subsequent seepage—I've seen beautiful fossils of pyrite which had been formed that way."
"But this shows no trace of structure, except for its exterior shape."
"Isn't a really well preserved structure the exception rather than the rule in fossils?"
"I suppose so. Still, I'd like to know just how far, and which way, this green thread goes. I'd also like to know whether there are dilute copper deposits spread through this rock, which could be concentrated in the way you suggest."
"The first could be learned by taking enough cores. The other would call for some very careful analysis of samples which had been selected with a very sedulous eye kept on the stratigraphy. You know that; you must have done that sort of thing looking for carbon-fourteen samples, at times."
"Yes, I see that. Could you make such analyses here?"
"No, except for the mere presence of copper. The cores would have to go back to a well equipped lab. Still, if you want to get them, it's all right with me. Problems were made to be solved. I'll admit this one doesn't seem very exciting to me, but I can use your data after you finish for work of my own. You should wind up with material for a pretty complete geochemical picture of this neighborhood. Shall I get the cores for you?"