In the present state of petrographical nomenclature the student 

 will be saved much trouble if he will fix his attention on the rocks 

 themselves instead of on the names which have been used in 

 different senses by different writers. 



In carrying out my work I have received great assistance 

 from many public institutions and private individuals. To the 

 authorities of the British Museum I am greatly indebted for 

 assistance in enabling me to examine the collections entrusted to 

 their charge. The Allport collection is especially valuable and 

 should be studied by all workers in British petrography. The 

 Director General of the Geological Survey kindly allowed me 

 to examine the large collection of slides of English rocks in the 

 Museum of Practical Geology. Prof. JUDD has frequently assisted 

 me, and I am indebted to him for the loan of slides from which 

 several figures have been prepared. I have to thank Mr. ALLPORT 

 for much kind help and for the loan of slides from his private 

 collection. Prof. BONNEY, who was my tutor at Cambridge, and 

 whose lectures were the means of inducing me to turn my 

 attention to the science of geology, has always been ready with 

 advice and similar assistance. Prof. HUGHES has aided me by the 

 loan of sections from the Woodwardian Museum. I am also greatly 

 indebted to many other friends, amongst whom I may mention : 

 Mr. BARROW, Prof. BLAKE, Mr. DAVIES, Mr. DICK, Prof. HULL, 

 Prof. LAPWORTH, Mr. MIERS, Mr. RUTLEY, Mr. WALLER and 

 Mr. WORTH. To the gentlemen above-named, and to all those 

 who have assisted me, I tender my warmest thanks. 



Dr. HATCH has kindly furnished me with the Glossary 

 which is printed after the text. He wishes me to state that it 

 has been arranged with special reference to the structure, both 

 macroscopic and microscopic, and mode of occurrence of rocks. 

 Mineralogical terms frequently used in describing the con- 

 stituents of rocks have been introduced, but rock-names have 

 been rigidly excluded. 



J. J. H. TEALL. 



12 , CUMBERLAND ROAD, KEW, 

 March I'M, 1888. 



