Geological Society, 219 



responding change, he says, takes place in the slate. The elvans 

 are connected on either side with the granite they intersect by the 

 most intimate mineral gradations, or contain irregular portions or 

 masses of the common granite, with which they coalesce ; both 

 are penetrated by crystals of felspar ; both are striped with shorl. 

 At Pednimerer Meer one of the parallel layers of granite runs 

 through the elvan. In Scotland different beds of granite will inter- 

 sect a common mass, and pass by minute mineral transition into one 

 another, or into the characteristic granite of the district. 



Dr. Boase considers that the Primitive Shisti are improperly said 

 to be stratified. Pini has expressed the same opinion in two sepa- 

 rate memoirs; the supposed planes of stratification are, in his view 

 of the subject, mere transverse fissures. Prof. Phillips, Mr. Scrope, 

 Dr. Fleming, Prof. Sedgwick, have all felt and expressed the dif- 

 ficulty of distinguishing in these shisti planes of cleavage, and planes 

 of stratification. In the days of my geological apprenticeship I took 

 great pains to observe and record dip and direction, and fondly 

 hoped to obtain so large a number of accurate data on this subject 

 as might enable me to arrive at last at some general and impor- 

 tant result. I threw these data into tables, which only bewildered 

 me. Suspecting the accuracy of my early observations, I re- 

 peated them again and again, guarding myself on every occasion 

 more and more against probable sources of error; still I found my 

 results perpetually varying, till at last my patience was exhausted, 

 my clinometer discarded, my registers destroyed. Let it not be 

 supposed, however, that my observations were useless ; they taught 

 me a salutary distrust. 



Dr. Boase remarks that all the Slate rocks are composed of rhom- 

 boidal concretions, which are developed on a large scale by disinte- 

 gration. Mr. Scrope had anticipated this remark, and generalizes it. 

 He says the stratification of rocks of all kinds, where the strata are 

 separated by seams, is produced by concretionary process. 



Now, then, which of all the planes are the planes of dip ? Dr. 

 Boase, like the Woodwardian Professor, selects those which run 

 with the laminae, and yet the layers of massive crystalline and 

 granitic rocks often lie the other way. But this seems to be very 

 much a matter of taste ; different observers selecting for the scene 

 of their measurements different planes. Some pay great attention to 

 the laminae, others neglect them ; nay, the same observer shall some- 

 times select as strata one series of planes, sometimes another. 



Professor Phillips, in a passage which is too long to be quoted, 

 has expressed the same idea in language equally expressive. 



Dr. Boase presents to those who differ from him on this subject 

 the following alternative : either Stratification implies a successive 

 deposition of sedimentary matters held in suspension, in which case 

 none of the primary shisti are stratified ; or merely parallel planes 

 without regard to the cause of their production, in which case not 

 only the primary shisti are stratified but granite also. 



In the thirteenth chapter will be found some excellent observations 

 on the nature of Inclined Strata, tending to show that these last are 



2F2 



