6 



J. A. PHILLIPS ON THE CONSTITUTION AND 



2. On the Constitution and History of Grits and Sandstones. By 

 J. Arthur Phillips, Esq., E.G.S. (Read December 15, 1880.) 



[Plates I. & II.] 



The careful and exhaustive researches relative to the constitution 

 and mode of formation of arenaceous rocks which have recently been 

 published by Professor Daubree, Dr. Sorby, and others, leave open a 

 comparatively restricted field for the pursuit of similar investiga- 

 tions. Having, however, during the last two years paid considerable 

 attention to the study of rocks of this class, I now venture to bring 

 before the notice of the Geological Society some facts and deductions 

 therefrom which would appear to have escaped the attention of 

 previous observers. 



Eor the convenience of developing certain ideas relative to this 

 subject, I propose in the present paper, in the first place, to describe 

 various grits and sandstones which have been microscopically and 

 otherwise examined. In doing this the older rocks will be con- 

 sidered first, and those of more recent age subsequently noticed in 

 the order of their geological sequence. The chemical composition 

 of some typical rocks will also be given. 



Secondly, the results of observations bearing on the effects pro- 

 duced by the action of flowing water on particles of sand and gravel 

 transported thereby will be described ; and finally, the more impor- 

 tant observed facts will be summarized,- and their bearings discussed. 



The difference between grit and sandstone is one not always 

 distinctly marked ; and numerous definitions of the two rocks, often 

 somewhat contradictory, have been given by various geologists at 

 different times. It has even been stated by an eminent authority 

 that rocks which in the north of England would be called grits, 

 receive the name of sandstones in the south*. 



In order, therefore, to avoid misunderstanding upon this point, 

 I may state that in the following descriptions the term grit is 

 applied only to coarse-grained arenaceous rocks of which the com- 

 ponent fragments are for the most part angular, and which, although 

 frequently crystalline in structure, seldom contain either perfect or 

 nearly perfect crystals. The cementing material of such rocks is, 

 as a rule, highly siliceous. 



Sandstones differ from grits in being finer in structure than the 

 latter, and in their component grains being usually less completely 

 incorporated with the cementing medium. The quartz in many 

 sandstones occurs principally in the form of perfect crystals, or in 

 that of crystalline aggregations. 



In quartzites the spaces between the component grains are com- 

 pletely filled by a siliceous cement, in which respect they closely 

 resemble some varieties of fine-grained grit. 



* 'Manual of Geology,' by John Phillips, F.R.S., p. 654. 



