In Chapter 5 of Silas Marner Silas gave a "cry of desolation." What does this mean?
The quote that you refer to actually comes in Chapter 5 of this great novel, so I have edited your question accordingly. It is in this chapter that Silas Marner returns to his cottage to find that his beloved gold has been taken from him. Because his gold was what he lived for and based his life around, its loss is an incredible blow to Silas Marner, and one which he struggles to accept:
Again he put his trembling hands to his head, and gave a wild ringing scream, the cry of desolation. For a few moments after, he stood motionless; the the cry had relived him from the first maddening pressure of the truth.
The word "desolation" can be defined as "the state of being abandoned or forsaken" or "wretchedness," and so we can see how Silas Marner felt having lost his gold. This has been his reason for existence for so long since he came to Raveloe, running away from broken relations in his past, and thus to have it taken from him so suddenly and unexpectedly would have made him feel forsaken and wretched.