Geoiogieai Syslems — Geological Maps, <^c. 253 



Art. VII. — Geological Systems. — Geological Maps. — C7i«- 



toyanl Feldspar. 



Extracts of letters to the Editor, dated at Paris January 10, 

 and March \ 4, from PFilliam Maclure, F. A. K. S. and 

 P. A. G. S. 



Your observation that some subdivision in the nomencla- 

 ture of rocks would be useful to geology, is perfectly Just, 

 although I doubt whether our present knowledge is adequate 

 to it Were we to judge by the diluvial and tertiary, these 

 doubts may perhaps be confirmed. The first is a division of 

 the alluvial class, without any distinct line of boundaries. 

 Whether a hill of sand, gravel, or clay, has been aggregated 

 by gravitation, from deep, still water, as a sea, lake, flood, &;c. 

 which they wish to call diluvial, or thrown together by the 

 action of a river, or the waves of the sea-shore, which they 

 wish to call alluvial, is' almost impossible to distinguish ; for 

 in both cases, the mass still remains a bed of sand, gravel, or 

 clay. The terrain de transporte of the French geologists, 

 applied to all rocks whose parts are rounded by attrition, in- 

 cludes almost the whole of Werner's alluvial. The tertiary, 

 including all rocks above the chalk, applies exactly to the 

 basin between France and England, from which the name 

 was perhaps derived, but cannot w^ll define a geological po- 

 sition where no chalk has been found. 



One may perhaps be warranted in supposing the existence 

 of a period, when there was, on the surface of the earth, only 

 the primitive, the oldest of the five classes of Werner's, the 

 other four classes appearing to be formed by fire, water, &;c. 

 out of the materials composing this first and most ancient 

 class. As we have not yet seen the laws of nature actually 

 operating, to form any rocks similar to the primitive, we are 

 left to conjecture the mode of formation ; the total absence nf 

 organic remains would lead to the supposition that it was con- 

 structed before their existence; the organic remains, and a 

 structure in the rocks similar to that which is actually exposed 

 to the evidence of our senses, warrants the supposition, that 

 the four other classes were formed at some period iii the pro- 

 gress of time. The primitive and transition have a fixed cha- 

 racter of universal origin in all parts of the globe where they 

 are found ; they have a regularity of stratification, inclination, 



