grammar - "Types of" followed by singular or plural?

When using the phrase "types of" or "kinds of," it often seems appropriate to follow with a singular noun (e.g., types of rock), but at other times a plural noun sounds better (e.g., types of sentences). Is there some kind of rule about this?

Answer

I think your essential concern is about countable vs. uncountable nouns. Countable nouns can be singular or plural; uncountable nouns are singular. Some words can be both, in different meanings.

Take a look at this example discussing countable and uncountable nouns:

The coffees I prefer are Arabica and Brazilian.
(Here coffees refers to different types of coffee)

You could write, "The types of coffee I prefer are Arabica and Brazilian."

So with your example:

The rocks I like are basalt and granite. [Rocks are countable]

The types of rock I like are basalt and granite. [Rock is uncountable]

When rock is taking on the uncountable meaning, it's not one rock - it's the idea of rock, the general concept. The only senses in which this is familiar to me are the "solid mineral" one and rock as in "rock music," where you might say, "The types of rock I like are grunge and punk."

Contrast the other part of your example:

The sentences I like are about geology. [Sentences are countable]

The types of sentences I like are about geology. [Sentences are still countable]

Sentences don't have an uncountable meaning:

*The types of sentence I like are about geology. [Ungrammatical for most speakers, I think]