Now that so close attention is being given to the microscopic investigation of rock structures, it is somewhat remarkable that no one should yet have questioned the validity of the views currently received regarding the exact constitution of limestone. Taking account only of those limestones whose original structure has not been obliterated by subsequent changes, the general view isthat nearly the whole mass, in the great majority of cases, is either directly, or indirectly, due to organic agencies. This view is even maintained in the cases where a microscopic examination of the rock fails to reveal more than a few traces of any structure that can be regarded as organic.