Finding an Average of every 25 rows.

Hi there, ive been given a large data set for a uni project which is too large to be useful at the minute. It consists of about 30,000 rows of element abundances for every 0.2mm of a sediment core. A sample of the file is attatched.

I would like to find the average of all the elements for every 5mm (so every 25 rows) of the sediment core with out having to go through every row individually finding an average. Ideally the average would be plotted against the medial depth of the values averaged i.e for the first 5mm the average would be plotted against 50.25cm.
Sorry if ive not given enough info (if not just let me know). Any help would be very much appreciated.