The Waikato contributes between 20 and 25 per cent of New Zealand's mineral production. Aggregate from Mesozoic rocks ranges from good (greywacke) to poor (argillite), with detailed differences being related to the position of the deposit within the New Zealand Geosyncline. Tertiary sediments show rapid facies changes that are reflected in the variability of important coal and limestone deposits. Petroleum and natural gas prospects are marginal at best. Upper Cenozoic deposits include sand, ironsand, pumice, perlite, aggregate, and building stone. Ground water is of vital importance, and is warm or hot in some areas. Good clays are available.