Thanks so much for that. It makes it that much more interesting to me to know something about it. I also learned that there is a Keweenaw Fault while I was Googling, not that it was involved. I don't think it's sandstone, as there aren't noticeable grains. It was found among sandstone, but it's much harder. Is there a way that I could tell if it were shale or mudstone? What would be the effect? I have HCl available. I spend a bit of time picking through the rocks on that shore every year, and I have never found one like it. I always keep an eye out. I've found a few agates, but never another one of these. I have found Petoskey's before in the LP.1) The angular (very angular) dark red to brown clasts, probably shale to slate or mudstone, to maybe sandstone if the grains are large enough
Off-white to light yellow matrix. The white matrix looks to be quartz which is a common form of silica – common in almost every rock. The other option is calcite, but calcite weathers and is usually softer, so soft that large rocks with a pure calcite matrix are far less common than quartz. Without scratching the rock, or the ability to drop dilute amounts or hydrochloric acid on it, I cannot tell if its quartz or calcite, but quarts seems the most reasonable.
I have no idea if this is a common local rock and if it has a local name – like the Petoskey stone as an example.
1) Sandstone and shale or mudstone often times have the similar composition - being dominantly quartz grains, but sandstone grains you can usually see, plus they are usually more beige to tan, orange or pink, not dark red/brown like this - think of Florida beach sand, that's clean sand waiting to be sandstone in a few million years. Shale/Mudstone is very very fine-grained quartz and other grains, mostly clay, that you cannot always see with the naked eye. The fact that you cannot see any little grains, leans me towards shale/mudstone or slate - which it also could be and is common in these age of rocks in the area. Slate is nothing more than a mudstone/shale that has gone through low-grade metamorphism. All this means is temperature and or pressure, beyond the normal burial and geothermal gradient has altered it by removing even more water and aligning the mineral grains. It becomes very hard and any grains are hard to see - pool tables have a slate underneath the felt. 2) Calite (Ca) is composed of calcium and calcium does not like to be alone, it always wants to be paired with something and its usually carbonate (CO3) in marine sedimentary environments. Together they form calcium carbonate CaCO3 and this is very basic - it's chalk, or limestone - the main ingredient in Tums or any antacid. Hydrocloric acid, is an acid obviously and when you combine an acid and a base, you get a reaction. If you were to drop 10% HCl on this rock and it began to fizz and stink a little, that is 100% diagnostic that it is calcium carbonate, the gas being released in the fizzing is carbon dioxide. Quartz is silica, made of silicon and silicon is very, very stable, it is extremely happy with oxygen as SiO2, it has no reason to do anything. This is why it is so common in rocks. It's extremely abundant and very stable. The chemical reaction is like the volcano that kids make in science classes with vinegar + baking soda, vinegar is acetic acid and baking soda is sodium bicarbonate, identical reaction from similar acid/base reaction. Sadly vinegar and calcium carbonate do not create the same reaction. It is a much slower reaction that takes days. You could take the rock and drop it a bowl of vinegar and in a few days it will dissolve if it's calcium carbonate, but then you'd be destroying it and that's not the point, a drop of HCl is harmless.
Seeing as it is quartz, then this is most likely a fault, or hydrothermal breccia. Quartz is extremely common so there are a few explanations, but here’s one that's reasonable given location. Quartz is soluble in water at high temperatures, like other elements and minerals, and water moves through everything in nature – even rocks. Think of table salt, when you boil water and add salt, the salt crystals dissolved faster in hot water. Upper Michigan has mining – copper and other metals – all related to the increased heat flow, therefore heated water from the Keweenawan Rift system, so that probably supplied the heat flow, which expelled hot water that allowed the mobilization of the quartz. As water moves over and over again over millions of years it will eventually cool or become super-saturated, which then precipitates the quartz, which then fills in cracks and broken fissures, encasing whatever rocks or fragments of rocks that are near – sometimes called veins. All this happened well below the surface, later it was exposed, eroded into a big chunk that got transported via water, broken up, rounded and smoothed along the way into this rock.