Combine both of the sentences by changing one of the sentences into a participial phrase.
Sentence: New England rock formations are like some in Britain. Their similarities intrigue geologists.
Is this right?: New England rocks are forming like the ones in Britain, their similarities intrigue geologists.
In this assignment you are asking your question about, you are requested to "Combine both of the sentences" by making one of the sentences into a participle phrase. Firstly, a participle phrase is a phrase that has a participle as a head word, not as an interior word, therefore a participle phrase must begin with a participle. Secondly, you must combine the two sentences to create one sentence and the combining lexeme must be a participle, not a conjunction nor a punctuation mark (such as a combining semicolon).
These are very strict and specific directions for which it is necessary to understand participles. There are two kinds of participles, a present -ing participle and a past -ed participle. [The -ing participle can be substituted by a present tense which-clause and the -ed participle can be substituted by a past tense which-clause.]
The correct way to combine these two sentences by means of a participle phrase is as follows: New England rock formations are like some in Britain, intriguing geologists. Intriguing is the combining present -ing participle that joins the sentences. [Substitute with which-clause: New England rock formations are like some in Britain, which intrigues geologists.]
Additionally, you will not want to change formations to are forming because formations is a noun and are forming is a present tense verb. In the assignment sentence, "New England rock formations are like some in Britain," a geological feature of antiquity is being discussed. In your suggested change, "New England rocks are forming like the ones in Britain," you are discussing a currently occurring process. You have thus changed the meaning and import of the sentence by shifting the stem word form- from the noun word class to the verb word class, thus creating an occurrence that would definitely intrigue geologists by changing the sentence Verb from the linking verb are ("formations are like") to the present tense verb are forming ("rocks are forming").
If you wish another option, after considerable manipulation of the sentences, it may be possible to join the sentences by a participle phrase in another way. The following highly manipulated construction is the other option: "New England rock formations intrigue geologists, being like some in Britain." Being is the present -ing participle that joins the sentences. [Substitute with which-clause: New England rock formations intrigue geologists, which are like some in Britain.] One caveat: It is possible to misunderstand this construction and think the geologists are equated with the geologists in Britain instead of the rock formations being equated.
Yes, and no, you are part of the way there by having the right idea.
What you did was change the noun formations into the participle forming from its verb "to form". What you didn't do was take the verb in the second sentence "to intrigue" and change it to a participle as well. The verb in the first sentence is "to be," its participle being "being." It would be difficult to re-word the first sentence to use the word "being," but much easier to change "to intrigue" to "intriguing."
A modified sentence would look like this:
New England rock formations are like some in Britain, their similarities are intriguing geologists.