While reading the book the following sentence made me interested. The usage of "as" is like the form "as....as" but not the same. + The comparison is made by a noun phrase(no difference) not adjectives or adverbs. I wonder whether this is a common usage or this is a special usage author chose?
It occurred to me that there was no difference between men, in intelligence or race, so profound "as" the difference between the sick and the well.(Great Gatsby)