Geologists Harris and Gass hypothesized that the Red Sea rift developed along the line of a suture (a splice in the Earth's crust) formed during the late Proterozoic era, and that significant observable differences in the composition of the upper layers of rocks deposited on either side of the suture give clues to the different natures of the underlying igneous rocks. Other geologists argued that neither the upper rock layer nor the underlying igneous rocks on the one side of the rift differ fundamentally from the corresponding layers on the other side. These geologists believe, therefore, that there is inadequate evidence to conclude that a suture underlies the rift. In response, Harris and Gass asserted that the upper rock layers on the two sides of the rift had not been shown to be of similar age, structure, or geochemical content. Furthermore, they cited new evidence that the underlying igneous rocks on either side of the rift contain significantly different kinds of rare metals.