How do geologist know what they are looking at really is what they say it is?

Yes. You might think, well, why can't you start with, for example, mud? But you can't get mud without starting with a rock, there's no process that just produces mud de novo.

Shale has lots of thin layers.

Well one factor is the proportion of chemical weathering on the one hand, and mechanical weathering and erosion on the other. Obviously mechanical weathering and erosion can produce chunks of pretty much any size.Chemical weathering, on the other hand ... consider a nice typical igneous rock such as granite. It will contain quartz, mica, amphibole, and two kinds of feldspar. Broadly speaking, the mica, amphibole and feldspar will be turned by chemical weathering into clay, which doesn't hold together particularly well, and which gives you fine particles: mud, in effect. The quartz, on the other hand, is indestructible, and gives us sand.(Because sand and clay particles are different sizes, they will be transported differently, which is why you end up with some places that are all sand and some which are all mud.)One consequence of this is that when there's been a lot of mechanical weathering involved in the production of sand the sand is arkose: that is, the grains contain an appreciable amount of feldspar; whereas when chemical weathering predominates the feldspar is converted to clay and the resulting sand is practically pure quartz.

Oh, I missed this. Slate is a metamorphic rock, and comes apart into layers because it has undergone foliation. To explain this, consider that the silicate minerals in a rock have shapes: they can come in strings or sheets. So when a rock is heated and compressed, these minerals get pushed so that they lie perpendicular to the direction of the compression.Have you read my book? All this stuff is in there; though I see the point of discussing these issues on this thread.

Yes indeed, that's just ordinary layering as found in sedimentary rock. Though it occurs to me that perhaps the fact that clay is a sheet silicates may also have something to do with the layering in shale as well: the sheet silicates would mean that clay often comes in flakes, which would tend to settle parallel to the ground and then would be forced even more into that orientation by compaction.Clay particles, magnified:

Sure, mechanical and chemical processes can obviously both operate on the same lump of rock. Often one will predominate: if, for example, a lump of granite is buried in the humid acidic flood of a rainforest, then that's going to be pretty much all chemical weathering. A cliff face will get pretty much all mechanical weathering and erosion. Yeah, mica is a sheet silicate, which explains why mica comes in sheets. I don't know much about asbestos, but wikipedia says that it can be formed from amphibole minerals, where the silicates form a double chain, which would explain its fibrous consistency; or from serpentine, which is a famly of minerals that can take on different crystal habits, but presumably the ones you make asbestos out of have chains or double chains.

Travertine is calcium carbonate deposited by hot springs. Chert is any fine-grained quartz, however deposited; hot springs can deposit it, but there are other ways. A clast by definition is a bit broken off a rock, so in general no, limestone (of which chalk is a subset) is formed from calcareous parts of organisms and so is a biochemical sedimentary rock.Of course, you can have rocks made of clasts of limestone, but they had to be produced by mechanical weathering and erosion of limestone, which had to be produced biochemically. Here's some limestone breccia (breccia because the clasts are big and unrounded). Yes. Lithification is the process of turning into stone; metamorphosis changes the properties of a stone.

And indeed the exact process. Metamorphosis has different results depending on the degree of temperature and pressure. (For example, I mentioned foliation above: this only happens when there is pressure, heat alone won't do it.) Or in sedimentation, to take a simple example, mudstone is produced by compaction by the weight of the overlying sediment/rock. Nw there is in fact continuum from loose wet mud to the hardest mudstone, and how hard it is will depend on how much it has been compacted.

Incidentally, can I have a quick question, I think I used to know it but I've forgotten. What do you call a conglomerate/breccia which is made of clasts from a previous conglomerate/breccia?

'Cos it tended to form in swamps by the shore; it was always just a little variation in sea level away from being in a marine environment.

Or, y'know, seascape. But yes, you get coal measures where you can see the same cycle over and over: paleosol (fossil soil) with root traces in it and tree stumps sticking out of it into coal which is then covered over by marine sediment, and then the same sequence over and over again.

It's the remains of part of a great big overturned fold. Here's a smaller one so you can see what's happening.

Oh yes. I wrote this article. Also if the bit that's been overturned is big enough you could use dating methods. Or, as in the photograph, the folds may still be visible.