6. It is thought that millions of years ago this liquid mass pushed upward through cracks in the earth's crust.
It is thought that…是一个很常用的句型，类似的句型在英语中很多，比如：
1) It is / was， has been…， reported that…
2) It is / was， has been…， stated that…
3) It is / was， has been…， pointed out that…
4) It is / was， has been…， mentioned that…
5) It is / was， has been…， believed that…
7. A. There are only four areas where very many diamonds have been formed.
B. The first known area was in India， where diamonds were found thousands of years ago.
定语从句可分为限定性定语从句和非限定性定语从句。A句是限定性定语从句，B句是非限定性定语从句。A、B两句中都是用关系副词where来引导定语从句，修饰表示地点的名词。请看下面的句子，注意限定性定语从句一般不用逗号隔开：
1) Is there a shop around where we can get some fruit? (附近有什么商店可以买到水果吗?)
2) This is the place where we met yesterday. (这是我们昨天碰头的地方。)
3) The small town， where he once worked， has turned to be a modern city. (他曾经工作过的那座小城，已经变成现代化的城市了。)
8. In the 1600's…(十七世纪)
In the 1720's…(十八世纪二十年代)
In the 1800's…(十九世纪)
请注意年代的表达方法，以上的年代也可以表达为in the 1600s; in the 1720s; in the 1800s.如果我们要表达“在十七世纪初(中、末)期”，则可以说in the early(mid， late) 1600's/1600s.
9. Diamonds became very popular with the kings and queens of Europe.
become / be popular with / among是一个常用词组，意思是“受…欢迎”，
10. India's supply of diamonds was finally running out after 2，500 yeas of mining the stones.
Supply在句子中做名词用，意思是“供应”。Supply常常可以做动词用，意思是“供给，提供”。
run out是一个常用词组，意思是“用完，用尽”。请看例句：
1) Time is running out. We have to hurry. (时间快到了，我们得快点。)
2) My patience is running out. (我快要耐不住了。)
3) His luck seemed to have run out. (他的好运似乎结束了。)
我们也可以用run out来表达类似的意思，但run out of的主语应该是“某人”。如：
1) They have run out of money， so they have to find a job. (他们钱用完了，所以得找一份工作。)
2) I have run out of ink. (我的墨水用完了。)
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