In recent years the technique of correlating sedimentary formations by means of their "heavy accessories" has made rapid progress. The method consists of crushing or otherwise disintegrating the rock and concentrating the scanty amounts of such minerals as garnet, ilmenite, magnetite, and zircon by settling in heavy liquids or by hydraulic classification. Any given stratigraphic horizon is likely to carry these min¬erals in roughly constant proportion and with constant size and shape of grain, a fact which can be used in recognizing the horizon wherever it is met in the district.

The same method has been extended to the correlation of igneous rocks. Here zircon is the most useful index mineral. Through comparison of the colors and shapes of zircon grains, two and possibly three different ages of granite have been distinguished among the pre-Cambrian intrusives of the Lake Superior Region.

Heavy mineral studies of pulverized churn drill sludges have been successful in identifying rock formations. At Kimberley, Nevada, where highly altered and mineralized sandstone, limestone and monzonite porphyry were difficult to distinguish from one another, the fol¬io wing quick test was devised: The sludge is panned and the heavier non-metallic section is scraped off to one side. The sulphides are then panned out and set aside. The heavy non-metallic fraction which consists of the coarser rock fragments and fine grained heavy minerals is somewhat further segregated by tapping on a card and then examined under the microscope. Sandstone shows detrital zircon in rounded grains; monzonite porphyry yields euhedral zircons with sharp crystal faces; limestone gives no zircon but commonly an abundance of con-tact metamorphic silicates. Identity of the zircon may be further confirmed under the ultraviolet light either with or without the use of the bin¬ocular microscope.