The Emergence of the Diversity of Igneous Rocks As A Geological Problem: Part One—Early Speculations

Speculation about igneous rock diversity began in the first half of the nineteenth century after acceptance of the existence of ancient volcanism and the recognition of two fundamental types of lava: basalt and trachyte. Before 1850, George Poulett Scrope (1797-1876), Charles Darwin (1809-1882), and James Dwight Dana (1813-1895) attributed diversity to intumescence of gas-rich lava, crystal settling, and differential fusion of minerals. In the 1850s, Robert Bunsen (1811-1899) maintained that lava is derived from two deep normal trachytic and normal pyroxenic sources. Wolfgang Sartorius von Waltershausen (1809-1876), Joseph Durocher (1817-1860), and Ferdinand von Richthofen (1833-1905) universalized Bunsen's sources by postulating a density-stratified Earth in which a layer of acid, feldspathic material rested above a layer of basic, basaltic material. Exploration of the complex volcanic terranes of western America in the 1860s and 1870s undermined the two-source theories and opened the way for the concept of fusion of already solid crust. Prior to 1880, speculations about diversity were typically suggested by naturalists, chemists, and geological generalists with strong interests in the geomorphic or geophysical aspects of Earth. Consequently, the problem of diversity was a peripheral concern to most of those proposing hypotheses. The hypotheses characteristically reflected the professional interests of their proposers. The content of the early speculations was further shaped by the nature of the field areas studied by proposers, and by their views on the correlation between geologic age and igneous rock type. Those, like Scrope, Darwin, Dana, Joseph Jukes (1811-1869), Carl Bernhard von Cotta (1808-1870), and Clarence Dutton (1841-1912), who rejected such correlations, located the source of igneous rock diversity at the surface, within a volcano, or within the acid crust. Those, like Bunsen, von Waltershausen, Durocher, von Richthofen, and Clarence King (1842-1901), who accepted the Wernerian idea that there had been changes in igneous rock type through time were more inclined to attribute diversity to multiple lava sources at great depth.

The Amphibolite of Doir'a'Chatha (Durcha), Sutherland

The amphibolite at Durcha passes from a dark, hornblende-rich rock, through every gradation of delicate striping and banding by quartzo-feldspathic material out into psammitic Moine schist, within a few metres. Irregular cross-cutting bands of quartz and feldspar which seem to “ feed ” the delicate stripes can be seen and there are intergrown mixtures of the dark and light components which recall appinites. Chemical and other evidence suggests that the dark rock is a concordant thpleiitic dolerite sill, whilst the macroscopic and microscopic evidence from the pale bands suggests that they are metasomatic in origin and connected with the widespread permeative alkali-injection of the Moine Schists. They seem to have been introduced into the sill along closely-spaced foliation planes parallel with those in the adjacent sediments and with the original contacts of the sill.