lisp question

I have a need for a matrix structure in lisp. I'm having
trouble defining how to handle this using lisp. The problem
is:
I have a list (X Y) which make up a matrix (M) with indices
i & j. I and J are unknown quantities before run time. The
matrix dimension of i x j is in excess of 10000. I must be
able to both add values to the end of the column and
retrieve columns of values to this structure. For example
M=
| 1 2 3 4 5 6 7
---------------
1 | 1 4 X 8 9 1 3
2 | 1 1 3 4 4 5
3 | 2 3 5 5 4 2
4 | 2 2 2 2 X 2
5 | 1 3 X X 0
6 | 0 X X
7 | X
Need to a save item to the end (or beginning) of any column
(where X's are shown) and need to retrieve the whole column
(not individual items).
Does anyone have an idea of how to handle this? I mentioned
the dimension of the Matrix so you have an understanding
that the structure must be fast. I cannot rebuild the list
each time I add an item.
My best plan to handle this was to use j+1 lists. There
would be one list for each column plus an index list
holding the names of the lists. For example the columns
would be named and in a list like this (col1 col2 col3...)
Where col1 was the var that pointed to the list. Then to
add to a list look-up the var then evaluate the name and
cons a new item to the list. To retrieve just look-up the
name. My problem here is I cannot figure out how to
generate the lists names (col1 etc) dynamically. (I don't
know the number of columns until run time.)
Any help would be appreciated.