How do you think my prospects are for grad school?

I know this is a very specific question without any real foreseeable answer, but any comforting advice would mean a lot to me.

I'm usually a very good student (G.P.A always above 3.0 in serious classes not just core classes) but this semester I was very overwhelmed and i anticipate getting D's in my chemistry and calculus classes. I'm not beating myself up over it because I worked fairly hard and I attend a fairly prestigious university and my calc professor told us the fail rate for his last semesters were around 24 F's to 2 B's.

I was wondering, as a Geology major, am I better off taking these classes over or is it ok if I leave them behind and just do well on the rest of my classes (like 400 level geology classes and such)? I don't mind going to a less than stellar grad school as long as I can get a higher education in Geology and ultimately get a job. But I want to know because I don't want to waste more time and money on classes that I honestly don't foresee mattering much in the long run. Chemistry yes, but I'll be taking higher levels and plan to do better in those.

Any advice would be appreciated...I'm getting queasy over this