Intensive study of the heavy accessory minerals and comparison of the suites of them occurring in igneous rocks has recently received much attention. In 1915 Rastall and Wilcockson examined the heavy accessories from granites of the English Lake District in connection with a study of the mineral constitution of certain sandstones. They found a wide variation in the accessory minerals of the granites, but a distinct similarity in those from all phases of the igneous mass.

Since 1876, when McCoy described two specimens of tuberculated plates under the name Asterolepis ornata var. australis, placoderm remains have been known to occur in the Middle Devonian marine limestones of Buchan, Victoria, Australia. Some years later, another specimen was obtained from the Buchan limestones by W. H. Ferguson of the Geological Survey of Victoria, and was referred by Chapman (1916) to the genus Phlyctaenaspis. Chapman, regarding McCoy's specimens as cospecific with the new material, included all the remains in the species Phlyctaenaspis australis (McCoy), distinguishing the more complete specimen discovered by Ferguson as P. australis var. confertituberculata Chapman. As Phlyctaenaspis is a form typical of the Lower Devonian, its reported occurrence in the Middle Devonian seemed to warrant further investigation. I therefore undertook a re-examination of all the available material, which is now preserved in the National Museum, Melbourne, and spent some months in removing the matrix from Ferguson's specimen. The matrix is a hard carbonaceous limestone containing remains of Spirifer yassensis and other invertebrates, and it was found that dilute hydrochloric acid would dissolve away this matrix without seriously affecting the fish plates, although care had to be taken not to subject these to too long contact with the acid. When as much of the matrix as possible had been removed mechanically by grinding, the fossil was exposed by brushing the dilute acid on to the remainder with a camel-hair brush, and continually observing the result under a binocular microscope.

In 1933 the present writer described and mapped an igneous complex in the counties of Tyrone and Derry in Northern Ireland, to which he gave the name of the Tyrone Igneous Series.

As early as 1915, it was made clear by Bowen that, given continuous separation of crystals from the successive residual liquids of a consolidating magma which was initially basaltic in composition, the inevitable result would be a gabbroic rock in depth, a granitic one above, and “various intermediate types in the intermediate layers”. As a necessary condition to the evolution of a magmatic residuum of granitic composition, this process of progressive crystallization differentiation involves the generation of intermediate rock-types from residual liquids of intermediate composition. Certain investigators, however, led by the authors of the Mull memoir (1924), have entirely overlooked the necessity of this condition. Faced with the association of contrasted acid and basic rocks, which is a conspicuous characteristic of many continental central complexes, they have attempted to account for the facts by introducing a modified conception of crystallization differentiation for which Nockolds has recently proposed the name contrasted differentiation. According to this conception, the residual liquid of consolidating basaltic magma is of granitic composition and can be separated in bulk from a crystalline gabbroic phase.