[R] selecting subsets of data from matrix

[Laura Quinn]

Probably a stupid question, but I don't seem to be able to find the answer
I'm looking for from any of the R literature. Basically I have a matrix
with several thousand rows and 20 columns(weather stations) of wind
direction data.

I am wanting to extract a matrix which contains data for all columns
conditional on column 20 having a value of _either_ less than 45 or
greater than 315. (ie I want to extract a matrix which contains wind direction
data for all columns {weather stations} when there is a prevailing
northerly wind for one of the stations).

I have tried a few different methods of doing this, none with any success,
can anyone please advise?

Thanks in advance!