While looking at the manuscripts, Agnes came across a very old one written in Syriac. The writing dated from the 8th century and appeared to be some form of ladies poetry. But what caught Agnes' attention was the faint words behind the poetry. In a very ancient hand was an older Syriac text that named the four Gospels. Agnes had discovered a palimpsest, a codex or scroll that had been scraped and washed clean and then written over again. It was an early form of recycling.
What Agnes had discovered was a "scraped" or "washed" manuscript of the Gospels from the fourth century. While the Syriac on top was 1100 years old, the faded words from the New Testament Bible below was 1500 years old. They had discovered our oldest copy of the Gospels written in Syriac now known as the Lewis Codex or Syriac Sinaiticus.
Janet Soskice from the University of Cambridge has written a book about these two women and their major discovery. It is called The Sisters of Sinai: How Two Lady Adventurers Discovered the Hidden Gospels. You can read the first chapter and some of the second on Amazon.
The Cambridge Faculty of Divinity web site has posted an 8 minute video interview with Soskice in which she describes the two sisters, their discovery and what it was like for women like them in the nineteenth century. They were more Indiana Jones than Harrison Ford.