322 W. M. Hutching s — The Origin of some States. 



all thicknesses alternate abruptly or gradually, according as the 

 conditions of deposit changed, in so many beds which had the same 

 origin throughout so far as we can see. 



1 hope I have also succeeded in showing that deposits of the waste 

 of granitic rocks by no means consist of inert substances that could 

 not readily undergo further change, but that, on the contrary, they 

 are very active indeed in such changes. 



It must be borne in mind, as Dr. Sorby points out, that it is more 

 than likely that in. many slates we have the result of deposits from 

 more than one source of material at the same time ; also that they 

 may be produced from different sources without such mixture. 

 Further, that the exact nature of slates is liable to vary very con- 

 siderably even in adjacent laminse, or in the same laminas at different 

 parts, so that care must be taken not to draw too decided conclusions 

 from one or two observations. 



In considering the nature of the mica, it is to be recollected 

 that some of these slates probably contain more or less of the 

 waste of still older slates of similar nature, which waste would 

 mainly consist of rutiliferous mica. Flakes of white, or almost 

 white mica, with the rutile inclosures, form a prominent con- 

 stituent of some silts and muds now being deposited, and of 

 some surface clays. This, however, would not much affect the 

 question of the first origin of the rutiliferous mica, and we may 

 reasonably suppose that in the early geological periods, to which the 

 slates now specially under discussion belong, there would be less 

 and less probability of previous slates supplying part of the material. 

 Of course, if my contention is correct, that many of the slates 

 resulted from alteration of sediments from granite, more or less rich 

 in biotite, and that these sediments were exactly similar in nature, 

 and underwent just the same processes of change as we can see in 

 the deposits of the Coal-measures, it is not in any way suggested 

 that the conditions of deposit were the same. These may have 

 varied to any extent as to depth of water, currents, etc., etc., but the 

 conditions of denudation would be more or less the same, and also 

 the nature of the finally deposited sediment. 



Shales, slates, and sandstones pass into one another by all degrees 

 of transition. Many sandstones, especially those of fine grain, have 

 as "cement" the same material which forms the "paste" in the 

 clays and shales above described, and frequently between the grains 

 of quartz we see films of just the same micaceous and rutiliferous 

 alteration-product. Mr. J. A. Phillips (Q.J.G-.S. vol. xxxvii.) seems 

 to have observed this in some cases. Thus he says (May Hill Sand- 

 stone) that the grains are " cemented together by a turbid siliceous 

 cement, suggesting the idea of its having been deposited from water 

 holding clay in suspension"; and again (Sandstone from Brigham 

 in Cumberland) he speaks of this cloudy cement and says, " Between 

 the fragments of this rock there are sometimes minute crystals of 

 a mineral which may be epidote." 



