Gradual cooling of the earth

You've managed to concentrate several errors into a single sentence.If the layers are sedimentary, cooling isn't a factor, at least not during sedimentary deposition. Sedimentary layers often become deeply buried and subject to great pressure, transforming them into rock. If buried deeply enough and the heat and pressure are great enough they're transformed into metamorphic rock (sedimentary rock that has been raised to melting or near melting temperatures), which will cool if the layers subsequently rise to the surface.Most sedimentary rock is the result of long periods of slow and fairly uniform deposition under relatively stable environmental conditions. As a result, most sedimentary layers are fairly uniform throughout the range of their thickness. For example, if you examine the layers of the Grand Canyon, you will not find the largest particles at the bottom of a layer and the smallest at the top. You'll find pretty much the same range of particle sizes at the bottom and at the top. If this weren't true then the layer wouldn't have the uniform appearance from top to bottom that is so common.If the layers are basalt, which is lava that has cooled, then be aware that basalt from a single event doesn't separate itself into layers as it cools. It's already fairly homogenous. The same is true when subterranean magma cools to form igneous rock. Granite is an igneous rock, and you can go to any granite quarry and discover that granite from a single locale is fairly uniform and has not separated into layers.--Percy