In connection with a study recently made of the famous tufa from Niangtzckuan, Dr. Barbour asked me to determine by analysis the relative amounts of soluble and insoluble matter so as to find out what proportion of the deposit is due to direct precipitation of mineral matter in solution and what amount is clastic material incorporated in the growing deposit. In connection with a study recently made of the famous tufa from Niangtzckuan, Dr. Barbour asked me to determine by analysis the relative amounts of soluble and insoluble matter so as to find out what proportion of the deposit is due to direct precipitation of mineral matter in solution and what amount is clastic material incorporated in the growing deposit.