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…是一個很常用的句型，類似的句型在英語中很多，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.

定語從句可分為限定性定語從句和非限定性定語從句。A句是限定性定語從句，B句是非限定性定語從句。A、B兩句中都是用關系副詞where來引導定語從句，修飾表示地點的名詞。請看下面的句子，注意限定性定語從句一般不用逗號隔開：

1) Is there a shop around where we can get some fruit? (附近有什么商店可以買到水果嗎?)

2) This is the place where we met yesterday. (這是我們昨天碰頭的地方。)

3) The small town， where he once worked， has turned to be a modern city. (他曾經工作過的那座小城，已經變成現代化的城市了。)

8. In the 1600's…(十七世紀)

In the 1720's…(十八世紀二十年代)

In the 1800's…(十九世紀)

請注意年代的表達方法，以上的年代也可以表達為in the 1600s; in the 1720s; in the 1800s.如果我們要表達“在十七世紀初(中、末)期”，則可以說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是一個常用詞組，意思是“受…歡迎”，

10. India's supply of diamonds was finally running out after 2，500 yeas of mining the stones.

Supply在句子中做名詞用，意思是“供應”。Supply常常可以做動詞用，意思是“供給，提供”。

run out是一個常用詞組，意思是“用完，用盡”。請看例句：

1) Time is running out. We have to hurry. (時間快到了，我們得快點。)

2) My patience is running out. (我快要耐不住了。)

3) His luck seemed to have run out. (他的好運似乎結束了。)

我們也可以用run out來表達類似的意思，但run out of的主語應該是“某人”。如：

1) They have run out of money， so they have to find a job. (他們錢用完了，所以得找一份工作。)

2) I have run out of ink. (我的墨水用完了。)