Quonset State (RI) airport (OQU) has class D airspace and is just
south of Providence (PVD), which has class C airspace.

The northern arc of the Class D airspace isn't charted with a
dashed blue line, so the best I can do is assume that this
boundary is coexistent with the Class C surface area. Some of
the class D airspace is under Class C with a floor of 1700 feet,
some under Class C with a floor of 1300 feet, and some is outside
the Class C airspace. To make things confusing on the chart, the
dividing line between the 1700' floor and 1300' floor is beneath
the [25] Class D ceiling symbol.

So according to the chart, the Class D airspace extends to 2500
feet. However, Quonset tower was sternly warning pilots the
other day that their Class D only went to 1300 feet and not to
exceed that without the blessing of Providence. So why does the
chart say 2500 feet for the Class D inside the Class C ring?

This airspace looks pretty confusing. For good measure there is
a class E surface area for a PVD approach under the Class C
ceiling inside the Class D ring, although we can assume that the
E is only in affect inside the D circle when Quonset tower is
closed. To be sure, the tower at Quonset keeps funny hours
somewhat (e.g. closed Mondays).