450 Alfred Bell — On the Butley Crag-pits. 



other metamorpliic action subsequently to their formation. In order 

 to acquire a knowledge of such facts, it is necessary to make a care- 

 ful study of rocks in various stages of alteration, — a study of great 

 interest, and of quite as much importance as the examination of 

 specimens supposed to be unaltered ; in fact, to select a typical speci- 

 men, and make a single section of it, is to leave off at the beginning 

 of the work. By making a number of thin sections from well- 

 selected specimens, a series may be obtained in which all the minerals 

 are to be seen in various stages of alteration ; and as some of the 

 changes are very charactaristic, it becomes possible to recognize the 

 original composition of a rock, although it may now differ completely 

 in appearance from its original state. 



Since I first discovered the invariable presence of olivine or its 

 pseudomorphs in the old basaltic rocks (melaphyres) of the Midland 

 Counties, I have extended my inquiries to the Scottish Coal-fields 

 and to other formations, and have obtained ample confirmation of 

 previous observations, as well as many additional facts. The evi- 

 dence appears to indicate very plainly that there is no essential 

 original difference between the true eruptive rocks of early geological 

 periods and those of recent or Tertiary age ; the later glassy and 

 trachytic rocks are closely analogous to the older pitchstones and 

 porphyrites, while the occurrence in Skye and other places of granites 

 and syenites of Tertiary age affords equally strong evidence in the 

 same direction. 



Satisfactory methods of investigation have been so recently em- 

 ployed, that very much remains to be done ; but it is already toler- 

 ably clear that the difference now found to exist between the so-called 

 Plutonic and the volcanic rocks is due to the metamorphic action to 

 which the former have been exposed during the long periods which 

 have elapsed since their original formation. 



IV. — The Butley Crag Pits. 

 By Alpred Bell. 



IN the July Number of the Geological Magazine a well-known 

 geologist^ has endeavoured to prove the existence of sauds of 

 the Chillesford age in the Crag Pits near Butley, referring more 

 especially to that on the Neutral Parm, near the Oyster Inn ; and as 

 I cannot agree with this opinion, I venture to lay before the readers 

 of the Magazine some of the notes upon which I ground my dissent. 



The accompanying figure represents the section now presented by 

 the excavation in question, which has been cut into a gently rising 

 slope for nearly 300 feet in length, by about 35 feet in its deepest 

 part. From the extreme right of the figure the side of the excava- 

 tion returns at nearly a right angle to the road, thus showing a cross 

 section. 



The top of the pit is covered with drift sand (full of minute 

 particles of quartz), containing a large number of rolled stones 



1 The Relation of the Bed to the Norwich Crag, by J. E. Taylor, F.G.S., Geol. 

 Mag., July, p. 314. 



