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valents of the Llandeilo flags and Caradoc sandstone. This sagacious 

 determination has since been confirmed by Mr. Salter as regards the 

 Garadoc sandstone ; the fossils of Bala and the typical Caradoc 

 sandstone of Sir Roderick Murchison in Shropshire being the same. 



The more elaborate paper of 1844 is accompanied by a geological 

 map of North Wales, and is less happy. Mr. Sharpe's genius 

 chiefly lay in the palseontological determination of the age of rocks, 

 and, in this case at least, the time he allowed himself to map North 

 Wales was too short for the satisfactory elucidation of the problems 

 he proposed to solve. 



Pursuing at intervals these subjects, Mr. Sharpe produced in 1847 

 an elaborate analysis and comparison of the Silurian fossils of North 

 America (collected by Sir Charles Lyell) with those of Great Britain, 

 and confirmed the views eutertained by the American geologist, Mr. 

 Hall, that the American Silurian strata, like the British, consist of 

 two great divisions, viz. Upper and Lower. 



While engaged in these investigations, Mr. Sharpe's attention was 

 drawn to the subject of slaty cleavage and foliation, which affects 

 the more ancient rocks of Devonshire, Wales, the North of England, 

 the Highlands of Scotland, and Mont Blanc. In 1846, 1848, 1852 

 and 1854 he produced four memoirs on these subjects, the two first 

 and the last of which are published in the Journal of the Geological 

 Society, and the third in the Philosophical Transactions. These 

 questions had previously been made the subject of special investiga- 

 tion by Professor Sedgwick, Mr. Darwin, and Professor Phillips. 

 It has been said that from imperfect data Mr. Sharpe generalized 

 too largely ; and though this may be the case, an attentive perusal of 

 the memoir of 1846 proves that in some important points he mate- 

 rially advanced the subject at that date in the direction to which the 

 labours of Mr. Sorby have since tended. He attributes the cleavage 

 of rocks, and consequent distortion of fossils, to pressure perpen- 

 dicular to the planes of cleavage, and asserts that rocks are expanded 

 along the cleavage planes in the direction of the dip of the cleavage. 

 In the communication of 1848, the doctrine that pressure is the 

 cause of cleavage is still more distinctly insisted on, and remarkable 

 instances are given in which pebbles were observed which appeared to 

 have been compressed and elongated in the planes of cleavage. He 

 also recognizes the fact, since so beautifully explained by Mr. Sorby, 



