THE 



GEOLOGICAL MAGAZINE. 



NEW SERIES. DECADE II. VOL. VI. 



No. XI.— NOVEMBER, 1879. 



OBIGIITAL AETICLES. 



I. — On the Eocks of Brazil Wood, Charnwoob Forest. 



By S. Allport, F.G.S. 

 N interesting discovery having just been made in consequence 

 of the examination of a single thin slice of rock, the following 

 communication is recommended to the attention of those geologists 

 who, to say the least, still fail to recognize the value of the micro- 

 scope in geological investigations. 



A short time since, a friend in Leicester kindly sent me a speci- 

 men of the so-called gneiss from Brazil Wood ; having prepared a 

 thin section and placed it under the microscope, I immediately re- 

 cognized an old acquaintance. It so closely resembled some of the 

 altered clay-slates which surround the Land's End mass of granite, 1 

 that I could not but regard it as another illustration of contact 

 metamorphism, and the following day was devoted to an examina- 

 tion of the locality in which it occurs. 



Under the guidance of Mr. W. J. Harrison, of Leicester, who kindly 

 accompanied me, a small quarry in the wood was soon found, and in 

 a few minutes we were gratified by the discovery of a mass of granite 

 in contact with the " gneiss." It is at present the only known 

 junction of the granite with the old sedimentary rocks, and proves 

 conclusively that, like the syenites of the district, the granite is also 

 an intrusive rock. Subsequently to our visit, Mr. Harrison has 

 discovered a bed of indurated banded slate overlying the gneiss ; it 

 has a distinct cleavage, and contains numerous small garnets. This 

 slaty rock is interbedded with another which is rather more com- 

 pact, and has a less perfect cleavage ; it also contains garnets. 

 The discovery of these slaty rocks is another point of interest, as 

 none have been previously observed to the east of the Swithland 

 slates. There can be no doubt that they belong to the Charnwood 

 series, and it will be seen that their altered condition may be fully 

 explained by their close proximity to the intrusive granite. As 

 the locality in which these discoveries have been made has been 

 frequently visited and described, it may afford satisfaction to previous 

 observers to learn that the granite just found has been exposed to 

 view by very recent quarrying of the rock. 



The microscopic structure of the Mount Sorrel granite, and of 



1 Described by me in Quart. Journ. Geol. Soc. 1876, vol. xxxii. p. 407. 



DECADE II. YOL. VI. — NO. XI. 31 



