Believe that of 怎么理解 system whose之间怎么有逗号呢

1.(be) believed to do 被认为是.或者：人们认为是.例如：He is believed to be the best singer of all.他被认为是众人中最好的歌手.
2.that of 中的that 是指的 material.that的这一用法是代指前面的可数名词单数或者不可数名词.我各举一例吧.The climate in Sichuan is nicer than that in Beijing.此处that代替同类的不可数名词climate.四川的气候比较北京的that好.（that指气候）.My book is different from that you have.（that=你的那本书.）
文章中是指,volcanic rocks are composed of material fundamentally different from that of the mid-ocean ridge system.（组成火山岩的物质不同于that of 大洋中背,也就是不同于大洋中背的物质,that指material物质）
3.whose source前后都有逗号,前面的逗号说明后面是一个非限制性定语从句,后面的逗号是因为插入了most geologists contend.(功能相当于 I think这样的插入语）.其实完整的句子是：whose source is the upper mantle.翻译成：其来源是上地幔.whose=of which

(which is ) believed to
that=material
whose引导非限制性从句，修饰 system