Use of comma

Hi, guys! How are you? I'm very glad to have joined the forum and I'm looking forward to learn with a lot with all of you. I'm a 20 years-old student from Brazil, and learning English is one of my favorite hobbies.

This week I was reading an entry from the Wikipedia (http://en.wikipedia.org/wiki/Medici_Chapel ) and was struck by a kind of doubt I'd never had before: if two sentences are related, but there's a comma separating them, to which part of the first sentence the second may refer to? To the whole of it? Is there a rule for these cases?

Here's the example:

"The Sagrestia Nuova was intended by Cardinal Giulio de' Medici and his cousin Pope Leo X as a mausoleum or mortuary chapel for members of the Medici family. It balances Brunelleschi's Sagrestia Vecchia, the "Old Sacristy" nestled between the left transept of San Lorenzo, with which it consciously competes, and shares its format of a cubical space surmounted by a dome, of gray pietra serena and whitewashed walls."

What do you think does the author mean here?

1) That the cubical space and the dome are made of gray pietra serena;

2) That only the dome is made of this material;

3) That only the cubical space is being referred to;

I'm more inclined to go for the first one, because, at least in theory, if the author wanted to refer to the dome only, he should not have put the comma, right?

Thank you a lot for your attention!

I hope this kind of question is welcomed in the forum!

Best Regards,

Laura.