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"While rock between two consistent strata might in one place be shale and in another sandstone, the fossils in that shale or sandstone were always the same."
I have trouble understanding the meaning of the word "consistent" here. Does it mean the two strata were firm and stuck together? Or does it mean the strata were the same ones in different places? Really need your help and thanks a lot!
The second meaning is your answer here. To more fully explain the sentence, 'consistent' applies to the dating of the strata in geological terms, implying that fossils identified to be of the same type which existed during the same period in time can be found in shale or sandstone, so long as the each type of stone can be shown to have the same geological age.
In your query sentence,
"While rock between two consistent strata might in one place be shale and in another sandstone, the fossils in that shale or sandstone were always the same,"
the description appears to involve a sandwich consisting of three layers — a top stratum or layer (let's call that A), a middle layer (B), and a bottom layer (C). Layers A and C are the 'consistent strata' — in other words, they are continuous, and their composition does not change.
The composition of the layer between A and C alternates between shale and sandstone as you move along the sandwich of strata, but the same fossils are found in this layer B, regardless of whether it happens to be composed of shale or sandstone at any given point.
NB — You will have noted that both shale and sandstone are a type of sedimentary rock, hence are formed in a broadly similar way.
