The North American frontier changed some of the characteristics of the pioneers of the 1750' s and intensified others. They were, as a group, semiliterate, proud, and stubborn, as dogged in their insistence on their own way of life as pine roots cracking granite ( 花岗岩) to grow. Perhaps their greatest resource was their capacity to endure. They outlasted recurrent ( 周期性发生的) plagues of smallpox (天花) and malaria (痢疾) and a steady progression of natural accidents. They were incredibly prolific. Squire Boone' s family of eight children was small by frontier standards. James Roberston, an eventful neighbor of Boone' s and the frontier of Nashville, had eleven children. Twice-married, John Sevier, the first governor of Tennessee, fathered eighteen his long time enemy, John Tipton, also twice married, produced seventeen. The entire assets of one of these huge families often amounted, in the beginning, to little more than an axe, a hunting knife, an auger, a rifle, a horse or two, some cattle and a few pigs, a sack of seed corn and another of salt, perhaps a crosscut saw, and a loom. Those who moved first into a new region lived for months at a time on wild meat, Indian maize, and native fruits in season. Yet if they were poor at the beginning, they confidently expected that soon they would be rich. In a way almost impossible to define to urban dwellers, a slice of ground suitable for farming represented not just dollars and cents, but dignity. The obsession brought shiploads of yearners (渴求者) every week to Boston, New York, Philadelphia, Baltimore, Charles Towne, and Savannah. It sent them streaming westward into the wilderness after their predecessors to raise still more children who wanted still more land. What kind of stylistic device did the author use in the phrase 'as pine roots cracking granite to grow'?