It has been objected that the subdivision of strata into zones has been pushed too far, but this is merely because in the establishment of zones, workers find it easier to work out the successive zones where the strata are thin and presumably deposited with extreme slowness, than where they are much thicker and have been rapidly accumulated, and accordingly, as the subdivision of strata into zones is a recent event, geological literature contains many more references to thin zones than to those of great thickness. Where an abundant and characteristic form (which is chosen as an index) of an assemblage of organic remains ranges through a great thickness of deposit, there is no objection to speaking of the whole as a zone, and it cannot be divided. To give some idea of the variations in the thickness of strata through which these abundant and characteristic forms will range, I append a list of the zones of graptolites which have been established amongst the Silurian rocks of English Lakeland and the thickness of each (which in the case of the thicker deposits is naturally only approximate):-- Thickness. Zone of Feet. Inches. _Monograptus leintwardinensis_ 5000 0 _Monograptus bohemicus_ 5000 0 _Monograptus Nilssoni_ 1000 0 _Cyrtograptus Murchisoni_ 1000 0 _Monograptus crispus_ 22 0 _Monograptus turriculatus_ 60 0 _Rastutes maximus_ 25 0 _Monograptus spinigerus_ 3 0 _Monograptus Clingani_ 3 0 _Monograptus convolutus_ 7 6 _Monograptus argenteus_ 0 8 _Monograptus fimbriatus_ 7 6 _Dimorphograptus confertus_ 25 0 _Diplograptus acuminatus_ 2 6 It must not be supposed that each of the subdivisions in the above list is of equal importance, and has occupied approximately the same length of time for its formation, but a study of the strata proves by various kinds of evidence that the deposits in which the characteristic forms range through a small thickness of rock were on the whole deposited much more slowly than where the range is continuous through a great thickness of deposit.