Carrol already said all I wanted to say about the distinction between "capitalism" as an object of study and some nebulous, trans-historical entity called "the economy".
I just want to address this from Dennis:
> The liberal-Victorian capitalism of the 1880s is a very different beast
> from today's transnational capitalism.
No doubt, but Marx's object of study was not a particular historical phase of capitalism, but rather, as he said himself in the preface, capitalism "at its ideal average". Marx deals with those aspects of capitalism that characterize it as such. No concrete analysis of capitalism looks exactly like the abstract capitalism in capital.
If you pay attention, the historical passages in Capital are illustrative of concepts. So the chapter on the working day comes after the chapter on absolute surplus-value. The chapters on manufacture and machine industry abound in historical examples, in order to illustrate the concept of relative surplus-value. The chapter on "so-called primitive accumulation" comes at the end of Chapter One to ground the necessary precondition of the doubly-free worker that must exist for the capital-relation to exist.
Now, it's up for question whether Marx actually succeeded in depicting a capitalism "at its ideal average". Some theorists seem to think Marx assumed that money must always have some money-commodity as its foundation, which as we know in the Post-Bretton Woods world isn't the case. But it was certainly his intent to provide such an analysis.