[R] using tapply with a matrix?

[Nurnberg-LaZerte]

I've successfully created my own function (topbot) that takes a vector as input and used it with tapply()  and an input vector (data$depth) and classification factor (data$profile):
vols <- tapply(data$depth, data$profile, topbot)
works great.

But now I want to do something similar, except my function will take a 3 column matrix with nrows() equal to the factor's length. tapply doesn't like this; I suspect it converts the 3 column matrox into a vector by ignoring the DIM atribute. 

I can't apply the funtion to each column separately as I need the info in all three simultaneously for the function's results.

I could always revert to "for" loops but was looking for a more concise solution in the spirit of "R".

Thanks in advance for any suggestions,
Bruce L.